Codepflege: CLAUDE.md und UI-Konsolidierung (Kapitel 13.4)

CLAUDE.md mit Projektkonventionen für künftige Claude-Code-Sitzungen.
Wiederkehrende UI-Muster konsolidiert: neue PageHeader-Control für
Titel/Untertitel in den Listen- und Detailansichten, globale Styles für
Dialog-Titel und Leerlisten-Hinweise statt inline wiederholter Werte.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

CLAUDE.md

@@ -0,0 +1,126 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project
+
+LehrerApp is a German-language desktop application for teachers (Notenverwaltung, Anwesenheit,
+Hausaufgaben, Klausuren, Unterrichtsplanung, Mitarbeitsbewertung). The primary product is the
+Avalonia desktop client backed by a local LiteDB file; an optional ASP.NET Core API enables
+multi-device sync via an encrypted event queue.
+
+The living roadmap and feature spec is `TODO.md` — check it before starting new work and update
+it (with an implementation note, following the existing style) when a checklist item is finished.
+`docs/Datenmodell.md` documents the meaning of domain fields where naming alone is ambiguous
+(e.g. why `GroupMembership` has no `SchoolYear` of its own).
+
+## Commands
+
+```bash
+# Build everything
+dotnet build LehrerApp.sln
+
+# Run all tests
+dotnet test LehrerApp.sln
+
+# Run one test project
+dotnet test LehrerApp.Tests/LehrerApp.Tests.csproj
+
+# Run a single test (xUnit fully-qualified name filter)
+dotnet test --filter "FullyQualifiedName~GradingServiceTests.RoundToGrade_Kaufmaennisch"
+
+# Run the desktop app
+dotnet run --project LehrerApp.Desktop
+
+# Run the sync API locally (needs JWT_SECRET — copy .env.example to .env first)
+dotnet run --project LehrerApp.Api
+```
+
+The API can also run via `docker/docker-compose.yml` (reads `JWT_SECRET` from the environment).
+
+## Architecture
+
+### Project layout
+
+- **LehrerApp.Core** — domain models (`Models/`), repository interfaces (`Interfaces/IRepositories.cs`),
+  and framework-free business services (`Services/`): `GradingService` (rounding, grading-key
+  validation, report-grade calculation), `SchoolYearService`, `AppLogger`, `BackupService`,
+  `AppLockService`. No dependency on LiteDB or Avalonia — this is what makes these services
+  testable without a UI or a real database.
+- **LehrerApp.Data** — LiteDB implementation. `LiteDbContext` owns the single `LiteDatabase`
+  connection and all collection accessors; `Repositories/AllRepositories.cs` implements every
+  `I*Repository` interface from Core. `DatabaseEncryptionService` handles password-protecting the
+  DB file.
+- **LehrerApp.Desktop** — the Avalonia MVVM client. `AppBootstrapper.BuildServices()` is the
+  single DI composition root (repositories and `LiteDbContext` are singletons — one LiteDB file
+  per user/process). `ViewModels/` and `Views/` are split into `Groups/`, `Students/`, `Settings/`
+  (mirrors the domain, not a strict 1:1 with models).
+- **LehrerApp.Sync** — sync client library used by Desktop: `EventQueue` (local outbox),
+  `ConflictResolver` (last-write-wins with timestamp tie-breaking), `SyncEngine` (push/pull
+  orchestration, timer-driven), `SnapshotService`, `Crypto/SyncCrypto` (AES-256-GCM payload
+  encryption — desktop events are encrypted at rest and in transit; Companion/WebApp events are
+  plaintext, see `PlainSyncEvent` vs `SyncEvent`).
+- **LehrerApp.Api** — minimal ASP.NET Core server: JWT auth, an append-only `EventStore` plus
+  `SnapshotStore`/`ReadableSnapshotStore` per device, mapped in `Endpoints/Endpoints.cs`. Sync is
+  optional — Desktop only registers `SyncEngine`/`SnapshotService` in DI when a server URL is
+  configured (`AppBootstrapper.LoadServerUrl`).
+- Each library has a matching `*.Tests` project (`LehrerApp.Tests` → Core, `LehrerApp.Data.Tests` →
+  Data, `LehrerApp.Desktop.Tests` → Desktop, `LehrerApp.Sync.Tests` → Sync), all xUnit.
+
+### MVVM conventions (Desktop)
+
+- ViewModels use CommunityToolkit.Mvvm source generators: `[ObservableProperty] private T _foo;`
+  and `[RelayCommand]`. A generated `partial void On{Foo}Changed(...)` is emitted for every
+  `[ObservableProperty]` — do not name a property/callback so it collides with that generated
+  name (e.g. a field named `OnFooChanged` next to `[ObservableProperty] Foo` fails with `CS0102`).
+- Field-level validation: each invalid input gets its own `{Field}Error` string property
+  (not one collective `ValidationMessage`), rendered directly under that field in XAML via
+  `IsVisible="{Binding {Field}Error, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"`.
+  `Save()` clears all `*Error` properties up front, accumulates failures into a local `valid`
+  flag instead of returning on the first one, then does `if (!valid) return;` — so multiple
+  errors can show at once. A message that only makes sense as one combined statement about
+  several fields (e.g. a weighting scheme's three percentages summing to 100) legitimately stays
+  a single message near those fields; that's a judgement call, not a bug.
+- Enum-backed UI: don't bind a `ComboBox` directly to a raw enum (Avalonia falls back to
+  `ToString()`, i.e. English member names). Expose a `string[] Options` of German labels plus a
+  `{Enum}Name` wrapper string property that the ComboBox binds to, converting to/from the actual
+  enum property — see `NiveauDisplay`/`GradeCategoryDisplay` for the established pattern.
+- Dialogs are `Window` subclasses shown via `await dialog.ShowDialog<bool>(owner)`, with
+  `Close(true)`/`Close(false)` on confirm/cancel and the result read from a `Result` property on
+  the dialog's ViewModel.
+
+### Avalonia XAML gotchas
+
+- Local attribute values always win over `Style` selectors for the same property. Conditional
+  `Classes`-based styling only works if the base value also comes from a `Style` selector, not a
+  local attribute on the element.
+- `[SomeAttached.Property] = value` is not valid inside a C# object initializer for an attached
+  property — use `ToolTip.SetTip(control, value)` as a separate statement.
+- `x:DataType` on a `Window`/`UserControl` enables compiled bindings, which validate binding paths
+  at build time — a clean build is meaningful evidence that new bindings are wired correctly, not
+  just that the XML parses.
+
+### LiteDB details
+
+- Only a property literally named `Id` is auto-recognized as the primary key. A model with a
+  differently-named identity property (e.g. `EventId`) needs an explicit `[BsonId]`, or LiteDB
+  silently assigns an unrelated `_id` and lookups/deletes by that property quietly do nothing.
+- `DateTime` round-trips through LiteDB BSON with `Kind` converted `Utc → Local` and the ticks
+  shifted to preserve the same instant. Since `DateTime` comparison operators compare raw ticks
+  and ignore `Kind`, comparing a round-tripped value against a freshly-created one is unsafe
+  outside UTC+0 — always `.ToUniversalTime()` both sides first (see `ConflictResolver`).
+  LiteDB's `LiteDatabase.Rebuild(new RebuildOptions { Password = ... })` for in-place re-encryption
+  is broken in the pinned LiteDB version (5.0.21) — it throws even against an unencrypted source
+  file. `DatabaseEncryptionService` instead copies every collection into a fresh database opened
+  with the target password.
+- `LiteDbContext` tracks a schema version in a `meta` collection (`RunVersionedMigrations`) so
+  migration steps run once, not on every startup. Add new migrations as another
+  `if (version < N) { ...; version = N; }` block rather than making old steps re-check their own
+  idempotency forever.
+- `new LiteDbContext(stream)` opens an in-memory database from a `MemoryStream` — used pervasively
+  in tests for a fast, disk-free `LiteDbContext`.
+
+### Central Package Management
+
+`Directory.Packages.props` pins all NuGet versions; individual `.csproj` files must reference
+packages without a `Version` attribute (`<PackageReference Include="Foo" />`).

LehrerApp.Desktop/App.axaml

@@ -5,5 +5,16 @@
   <Application.Styles>
     <FluentTheme />
     <StyleInclude Source="avares://Avalonia.Controls.DataGrid/Themes/Fluent.xaml"/>
+
+    <!-- Wiederkehrende Textmuster (13.4.3): Dialog-Titel und Leerlisten-Hinweis wurden bisher
+         in jeder View einzeln mit denselben Werten inline gesetzt. -->
+    <Style Selector="TextBlock.dialogtitle">
+      <Setter Property="FontSize" Value="18"/>
+      <Setter Property="FontWeight" Value="SemiBold"/>
+    </Style>
+    <Style Selector="TextBlock.emptyhint">
+      <Setter Property="Opacity" Value="0.4"/>
+      <Setter Property="FontSize" Value="13"/>
+    </Style>
   </Application.Styles>
 </Application>

LehrerApp.Desktop/ViewModels/Students/StudentViewModels.cs

@@ -18,6 +18,7 @@ public partial class StudentListViewModel : ObservableObject
     [ObservableProperty] private StudentListItem? _selectedStudent;
 
     public ObservableCollection<StudentListItem> Students { get; } = [];
+    public string CountSummary => $"{Students.Count} Schüler gesamt";
 
     public StudentListViewModel(IStudentRepository students)
     {
@@ -40,6 +41,7 @@ public partial class StudentListViewModel : ObservableObject
             : all.Where(s => s.LastName.Contains(SearchText, StringComparison.OrdinalIgnoreCase)
                           || s.FirstName.Contains(SearchText, StringComparison.OrdinalIgnoreCase));
         foreach (var s in f) Students.Add(new StudentListItem(s));
+        OnPropertyChanged(nameof(CountSummary));
     }
 
     public Func<Task>? OnAddStudent { get; set; }

LehrerApp.Desktop/Views/Dashboard/DashboardView.axaml

@@ -38,7 +38,7 @@
                 </DataTemplate>
               </ItemsControl.ItemTemplate>
             </ItemsControl>
-            <TextBlock Text="Keine Stunden heute" Opacity="0.4" FontSize="13"
+            <TextBlock Text="Keine Stunden heute" Classes="emptyhint"
                        IsVisible="{Binding !TodaysLessons.Count}"/>
           </StackPanel>
         </Border>
@@ -62,7 +62,7 @@
                 </DataTemplate>
               </ItemsControl.ItemTemplate>
             </ItemsControl>
-            <TextBlock Text="Keine offenen Aufgaben" Opacity="0.4" FontSize="13"
+            <TextBlock Text="Keine offenen Aufgaben" Classes="emptyhint"
                        IsVisible="{Binding !OpenTasks.Count}"/>
           </StackPanel>
         </Border>
@@ -195,7 +195,7 @@
                 </DataTemplate>
               </ItemsControl.ItemTemplate>
             </ItemsControl>
-            <TextBlock Text="Keine offenen Entschuldigungen." Opacity="0.4" FontSize="13"
+            <TextBlock Text="Keine offenen Entschuldigungen." Classes="emptyhint"
                        IsVisible="{Binding !OpenExcuses.Count}"/>
           </StackPanel>
         </Border>
@@ -228,7 +228,7 @@
                 </DataTemplate>
               </ItemsControl.ItemTemplate>
             </ItemsControl>
-            <TextBlock Text="Noch keine Lerngruppen." Opacity="0.4" FontSize="13"
+            <TextBlock Text="Noch keine Lerngruppen." Classes="emptyhint"
                        IsVisible="{Binding !CurrentGroups.Count}"/>
           </StackPanel>
         </Border>

LehrerApp.Desktop/Views/Groups/AddGroupDialog.axaml

@@ -9,7 +9,7 @@
 
   <Grid RowDefinitions="*,Auto" Margin="24">
     <StackPanel Grid.Row="0" Spacing="14">
-      <TextBlock Text="{Binding DialogTitle}" FontSize="18" FontWeight="SemiBold"/>
+      <TextBlock Text="{Binding DialogTitle}" Classes="dialogtitle"/>
 
       <!-- Typ: Klasse oder Kurs -->
       <StackPanel Spacing="4">

LehrerApp.Desktop/Views/Groups/AddSessionDialog.axaml

@@ -9,7 +9,7 @@
 
   <Grid RowDefinitions="*,Auto" Margin="24">
     <StackPanel Grid.Row="0" Spacing="14">
-      <TextBlock Text="Neuer Bewertungszeitpunkt" FontSize="18" FontWeight="SemiBold"/>
+      <TextBlock Text="Neuer Bewertungszeitpunkt" Classes="dialogtitle"/>
 
       <StackPanel Spacing="4">
         <TextBlock Text="Datum *" FontSize="12" Opacity="0.7"/>

LehrerApp.Desktop/Views/Groups/AddStudentToGroupDialog.axaml

@@ -11,7 +11,7 @@
 
     <!-- Überschrift + Suche -->
     <StackPanel Grid.Row="0" Spacing="12" Margin="0,0,0,12">
-      <TextBlock Text="Schüler zur Gruppe hinzufügen" FontSize="18" FontWeight="SemiBold"/>
+      <TextBlock Text="Schüler zur Gruppe hinzufügen" Classes="dialogtitle"/>
       <TextBox Text="{Binding SearchText}"
                PlaceholderText="Schüler suchen …"
                x:Name="SearchBox"/>

LehrerApp.Desktop/Views/Groups/CollectiveGradeDialog.axaml

@@ -9,7 +9,7 @@
 
   <Grid RowDefinitions="Auto,Auto,*,Auto" Margin="24">
 
-    <TextBlock Grid.Row="0" Text="Sammelnote erfassen" FontSize="18" FontWeight="SemiBold"/>
+    <TextBlock Grid.Row="0" Text="Sammelnote erfassen" Classes="dialogtitle"/>
 
     <StackPanel Grid.Row="1" Spacing="6" Margin="0,12,0,10">
       <Grid ColumnDefinitions="120,*,90" ColumnSpacing="6">

LehrerApp.Desktop/Views/Groups/DeleteExamDialog.axaml

@@ -7,7 +7,7 @@
         CanResize="False" WindowStartupLocation="CenterOwner">
   <Grid RowDefinitions="*,Auto" Margin="24">
     <StackPanel Grid.Row="0" Spacing="12">
-      <TextBlock Text="Klausur wirklich löschen?" FontSize="18" FontWeight="SemiBold"/>
+      <TextBlock Text="Klausur wirklich löschen?" Classes="dialogtitle"/>
       <TextBlock Text="{Binding}" FontSize="15" FontWeight="SemiBold" TextWrapping="Wrap"/>
       <TextBlock Text="Dabei werden auch alle erfassten Ergebnisse dieser Klausur dauerhaft gelöscht."
                  TextWrapping="Wrap" Opacity="0.7"/>

LehrerApp.Desktop/Views/Groups/DeleteGroupDialog.axaml

@@ -7,7 +7,7 @@
         CanResize="False" WindowStartupLocation="CenterOwner">
   <Grid RowDefinitions="*,Auto" Margin="24">
     <StackPanel Grid.Row="0" Spacing="12">
-      <TextBlock Text="Lerngruppe wirklich löschen?" FontSize="18" FontWeight="SemiBold"/>
+      <TextBlock Text="Lerngruppe wirklich löschen?" Classes="dialogtitle"/>
       <TextBlock Text="{Binding}" FontSize="15" FontWeight="SemiBold" TextWrapping="Wrap"/>
       <TextBlock Text="Dabei werden auch Einschreibungen, Klausuren, Noten, Unterrichtsplanung und Mitarbeitseinträge dieser Lerngruppe dauerhaft gelöscht. Wenn du die Gruppe nur ausblenden möchtest, nutze stattdessen das Archiv."
                  TextWrapping="Wrap" Opacity="0.7"/>

LehrerApp.Desktop/Views/Groups/ExamDialog.axaml

@@ -12,7 +12,7 @@
     <ScrollViewer Grid.Row="0">
       <StackPanel Spacing="14" Margin="0,0,12,0">
         <StackPanel Spacing="2">
-          <TextBlock Text="{Binding DialogTitle}" FontSize="18" FontWeight="SemiBold"/>
+          <TextBlock Text="{Binding DialogTitle}" Classes="dialogtitle"/>
           <TextBlock Text="{Binding SubjectDisplay}" FontSize="12" Opacity="0.6"/>
         </StackPanel>
 

LehrerApp.Desktop/Views/Groups/ExamEvaluationDialog.axaml

@@ -13,7 +13,7 @@
 
         <StackPanel Spacing="2">
           <StackPanel Orientation="Horizontal" Spacing="8">
-            <TextBlock Text="{Binding ExamTitle}" FontSize="18" FontWeight="SemiBold" VerticalAlignment="Center"/>
+            <TextBlock Text="{Binding ExamTitle}" Classes="dialogtitle" VerticalAlignment="Center"/>
             <Border Background="{DynamicResource SystemControlBackgroundAccentBrush}" CornerRadius="4" Padding="8,2"
                     IsVisible="{Binding NiveauLabel, Converter={x:Static StringConverters.IsNotNullOrEmpty}}">
               <TextBlock Text="{Binding NiveauLabel}" FontSize="12" Foreground="White"/>
@@ -70,7 +70,7 @@
             </DataTemplate>
           </ItemsControl.ItemTemplate>
         </ItemsControl>
-        <TextBlock Text="Noch keine bewerteten Ergebnisse." Opacity="0.4" FontSize="13"
+        <TextBlock Text="Noch keine bewerteten Ergebnisse." Classes="emptyhint"
                    IsVisible="{Binding !GradedCount}"/>
 
         <Separator/>

LehrerApp.Desktop/Views/Groups/GroupDetailView.axaml

@@ -3,6 +3,7 @@
              xmlns:vm="clr-namespace:LehrerApp.Desktop.ViewModels.Groups"
              xmlns:views="clr-namespace:LehrerApp.Desktop.Views.Groups"
              xmlns:models="clr-namespace:LehrerApp.Core.Models;assembly=LehrerApp.Core"
+             xmlns:shared="clr-namespace:LehrerApp.Desktop.Views.Shared"
              x:Class="LehrerApp.Desktop.Views.Groups.GroupDetailView"
              x:DataType="vm:GroupDetailViewModel">
 
@@ -13,10 +14,7 @@
             BorderBrush="{DynamicResource SystemControlForegroundBaseLowBrush}"
             BorderThickness="0,0,0,1">
       <Grid ColumnDefinitions="*,Auto">
-        <StackPanel Grid.Column="0">
-          <TextBlock Text="{Binding GroupTitle}" FontSize="22" FontWeight="SemiBold"/>
-          <TextBlock Text="{Binding GroupSubtitle}" FontSize="12" Opacity="0.5"/>
-        </StackPanel>
+        <shared:PageHeader Grid.Column="0" Title="{Binding GroupTitle}" Subtitle="{Binding GroupSubtitle}"/>
         <StackPanel Grid.Column="1" Orientation="Horizontal" Spacing="8">
           <TextBlock VerticalAlignment="Center" Opacity="0.6" FontSize="13">
             <Run Text="{Binding StudentCount}"/>

LehrerApp.Desktop/Views/Groups/GroupListView.axaml

@@ -1,6 +1,7 @@
 <UserControl xmlns="https://github.com/avaloniaui"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              xmlns:vm="clr-namespace:LehrerApp.Desktop.ViewModels.Groups"
+             xmlns:shared="clr-namespace:LehrerApp.Desktop.Views.Shared"
              x:Class="LehrerApp.Desktop.Views.Groups.GroupListView"
              x:DataType="vm:GroupListViewModel">
 
@@ -11,10 +12,7 @@
             BorderBrush="{DynamicResource SystemControlForegroundBaseLowBrush}"
             BorderThickness="0,0,0,1">
       <Grid ColumnDefinitions="*,Auto,Auto">
-        <StackPanel Grid.Column="0">
-          <TextBlock Text="Lerngruppen" FontSize="22" FontWeight="SemiBold"/>
-          <TextBlock Text="{Binding ListSummary}" FontSize="12" Opacity="0.5"/>
-        </StackPanel>
+        <shared:PageHeader Grid.Column="0" Title="Lerngruppen" Subtitle="{Binding ListSummary}"/>
         <ComboBox Grid.Column="1" ItemsSource="{Binding SchoolYears}"
                   SelectedItem="{Binding SelectedSchoolYear}"
                   Width="100" Margin="0,0,8,0" VerticalAlignment="Center"/>

LehrerApp.Desktop/Views/Groups/ParticipationGradeDialog.axaml

@@ -9,7 +9,7 @@
 
   <Grid RowDefinitions="Auto,Auto,Auto,*,Auto" Margin="24">
 
-    <TextBlock Grid.Row="0" Text="Mitarbeitsnote berechnen" FontSize="18" FontWeight="SemiBold"/>
+    <TextBlock Grid.Row="0" Text="Mitarbeitsnote berechnen" Classes="dialogtitle"/>
 
     <Grid Grid.Row="1" ColumnDefinitions="Auto,Auto,*" Margin="0,12,0,10">
       <TextBlock Grid.Column="0" Text="Zeitraum:" VerticalAlignment="Center" Margin="0,0,8,0"/>

LehrerApp.Desktop/Views/Groups/ParticipationWizardDialog.axaml

@@ -13,7 +13,7 @@
     <StackPanel Grid.Row="0" Spacing="2">
       <TextBlock Text="{Binding GroupLabel}" FontSize="13" Opacity="0.5"/>
       <Grid ColumnDefinitions="Auto,*,Auto">
-        <TextBlock Grid.Column="0" Text="{Binding StudentName}" FontSize="18" FontWeight="SemiBold"/>
+        <TextBlock Grid.Column="0" Text="{Binding StudentName}" Classes="dialogtitle"/>
         <StackPanel Grid.Column="2" Orientation="Horizontal" Spacing="8">
           <Button Content="◂ Zurück" Command="{Binding PreviousStudentCommand}"/>
           <TextBlock Text="{Binding ProgressText}" VerticalAlignment="Center" Opacity="0.6" FontSize="12"/>

LehrerApp.Desktop/Views/Groups/ReportGradeDialog.axaml

@@ -10,7 +10,7 @@
   <Grid RowDefinitions="Auto,Auto,*,Auto" Margin="24">
 
     <StackPanel Grid.Row="0" Spacing="2">
-      <TextBlock Text="{Binding GroupLabel}" FontSize="18" FontWeight="SemiBold"/>
+      <TextBlock Text="{Binding GroupLabel}" Classes="dialogtitle"/>
       <TextBlock Text="{Binding SchemeSummary}" FontSize="12" Opacity="0.6"/>
     </StackPanel>
 

LehrerApp.Desktop/Views/Groups/StudentGradesDialog.axaml

@@ -9,7 +9,7 @@
 
   <Grid RowDefinitions="Auto,Auto,*,Auto" Margin="24">
 
-    <TextBlock Grid.Row="0" Text="{Binding StudentName}" FontSize="18" FontWeight="SemiBold"/>
+    <TextBlock Grid.Row="0" Text="{Binding StudentName}" Classes="dialogtitle"/>
 
     <Button Grid.Row="1" Content="＋ Note hinzufügen" Command="{Binding AddEntryCommand}"
             HorizontalAlignment="Left" Margin="0,10,0,10"/>
@@ -52,7 +52,7 @@
       </ItemsControl>
     </ScrollViewer>
 
-    <TextBlock Grid.Row="2" Text="Noch keine Noten erfasst." Opacity="0.4" FontSize="13"
+    <TextBlock Grid.Row="2" Text="Noch keine Noten erfasst." Classes="emptyhint"
                HorizontalAlignment="Center" VerticalAlignment="Top" Margin="0,20,0,0"
                IsVisible="{Binding !Entries.Count}"/>
 

LehrerApp.Desktop/Views/Settings/GradingKeyTemplateDialog.axaml

@@ -10,7 +10,7 @@
   <Grid RowDefinitions="*,Auto" Margin="24">
     <StackPanel Grid.Row="0" Spacing="10">
       <StackPanel>
-        <TextBlock Text="{Binding Name}" FontSize="18" FontWeight="SemiBold"/>
+        <TextBlock Text="{Binding Name}" Classes="dialogtitle"/>
         <TextBlock Text="{Binding GradingSystemLabel}" FontSize="12" Opacity="0.5"/>
       </StackPanel>
 

LehrerApp.Desktop/Views/Settings/SettingsView.axaml

@@ -1,13 +1,13 @@
 <UserControl xmlns="https://github.com/avaloniaui"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              xmlns:vm="clr-namespace:LehrerApp.Desktop.ViewModels.Settings"
+             xmlns:shared="clr-namespace:LehrerApp.Desktop.Views.Shared"
              x:Class="LehrerApp.Desktop.Views.Settings.SettingsView"
              x:DataType="vm:SettingsViewModel">
 
   <Grid RowDefinitions="Auto,*">
 
-    <TextBlock Grid.Row="0" Text="Einstellungen" FontSize="22" FontWeight="SemiBold"
-               Margin="32,28,32,0"/>
+    <shared:PageHeader Grid.Row="0" Title="Einstellungen" Margin="32,28,32,0"/>
 
     <TabbedPage Grid.Row="1" TabPlacement="Top">
 
@@ -88,7 +88,7 @@
                        IsVisible="{Binding CatalogValidation, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
 
             <!-- Leerer Zustand -->
-            <TextBlock Text="Kein Fach ausgewählt." Opacity="0.4" FontSize="13"
+            <TextBlock Text="Kein Fach ausgewählt." Classes="emptyhint"
                        IsVisible="{Binding CatalogSubject, Converter={x:Static ObjectConverters.IsNull}}"/>
 
             <!-- Domain-Liste -->
@@ -166,7 +166,7 @@
         <ScrollViewer>
           <StackPanel Margin="32,20,32,28" Spacing="14" MaxWidth="640">
 
-            <TextBlock Text="Noch keine Vorlagen angelegt." Opacity="0.4" FontSize="13"
+            <TextBlock Text="Noch keine Vorlagen angelegt." Classes="emptyhint"
                        IsVisible="{Binding !GradingKeyTemplateList.Count}"/>
 
             <ItemsControl ItemsSource="{Binding GradingKeyTemplateList}">
@@ -298,7 +298,7 @@
                   </DataTemplate>
                 </ItemsControl.ItemTemplate>
               </ItemsControl>
-              <TextBlock Text="Noch keine Backups vorhanden." Opacity="0.35" FontSize="12"
+              <TextBlock Text="Noch keine Backups vorhanden." Classes="emptyhint"
                          IsVisible="{Binding !Backups.Count}"/>
             </StackPanel>
 

LehrerApp.Desktop/Views/Shared/ConfirmDialog.axaml

@@ -7,7 +7,7 @@
         CanResize="False" WindowStartupLocation="CenterOwner">
   <Grid RowDefinitions="*,Auto" Margin="24">
     <StackPanel Grid.Row="0" Spacing="12">
-      <TextBlock Text="{Binding Title}" FontSize="18" FontWeight="SemiBold"/>
+      <TextBlock Text="{Binding Title}" Classes="dialogtitle"/>
       <TextBlock Text="{Binding Message}" TextWrapping="Wrap" Opacity="0.8"/>
     </StackPanel>
     <Grid Grid.Row="1" ColumnDefinitions="*,8,*" Margin="0,20,0,0">

LehrerApp.Desktop/Views/Shared/PageHeader.axaml

@@ -0,0 +1,13 @@
+<UserControl xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             x:Class="LehrerApp.Desktop.Views.Shared.PageHeader"
+             x:CompileBindings="False">
+  <StackPanel>
+    <TextBlock Text="{Binding Title, RelativeSource={RelativeSource AncestorType=UserControl}}"
+               FontSize="22" FontWeight="SemiBold"/>
+    <TextBlock Text="{Binding Subtitle, RelativeSource={RelativeSource AncestorType=UserControl}}"
+               FontSize="12" Opacity="0.5"
+               IsVisible="{Binding Subtitle, RelativeSource={RelativeSource AncestorType=UserControl},
+                           Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
+  </StackPanel>
+</UserControl>

LehrerApp.Desktop/Views/Shared/PageHeader.axaml.cs

@@ -0,0 +1,29 @@
+using Avalonia;
+using Avalonia.Controls;
+
+namespace LehrerApp.Desktop.Views.Shared;
+
+/// Seiten-/Detail-Kopfzeile (Titel + optionaler Untertitel) — bisher in jeder
+/// Listen-/Detailansicht einzeln mit denselben Werten nachgebaut (13.4.3).
+public partial class PageHeader : UserControl
+{
+    public static readonly StyledProperty<string> TitleProperty =
+        AvaloniaProperty.Register<PageHeader, string>(nameof(Title), defaultValue: "");
+
+    public static readonly StyledProperty<string?> SubtitleProperty =
+        AvaloniaProperty.Register<PageHeader, string?>(nameof(Subtitle));
+
+    public string Title
+    {
+        get => GetValue(TitleProperty);
+        set => SetValue(TitleProperty, value);
+    }
+
+    public string? Subtitle
+    {
+        get => GetValue(SubtitleProperty);
+        set => SetValue(SubtitleProperty, value);
+    }
+
+    public PageHeader() => InitializeComponent();
+}

LehrerApp.Desktop/Views/Students/AddStudentDialog.axaml

@@ -12,7 +12,7 @@
     <ScrollViewer Grid.Row="0" VerticalScrollBarVisibility="Auto">
       <StackPanel Spacing="14">
 
-        <TextBlock Text="Neuen Schüler anlegen" FontSize="18" FontWeight="SemiBold"/>
+        <TextBlock Text="Neuen Schüler anlegen" Classes="dialogtitle"/>
 
         <!-- Name -->
         <Grid ColumnDefinitions="*,10,*">
@@ -94,7 +94,7 @@
           </ItemsControl>
 
           <TextBlock Text="Noch keine Kontakte. Über ＋ Kontakt hinzufügen."
-                     Opacity="0.35" FontSize="12"
+                     Classes="emptyhint"
                      IsVisible="{Binding !Contacts.Count}"/>
         </StackPanel>
 

LehrerApp.Desktop/Views/Students/ContactEditDialog.axaml

@@ -11,7 +11,7 @@
   <Grid RowDefinitions="*,Auto" Margin="24,20">
     <ScrollViewer Grid.Row="0" VerticalScrollBarVisibility="Auto">
       <StackPanel Spacing="16">
-        <TextBlock Text="{Binding DialogTitle}" FontSize="18" FontWeight="SemiBold"/>
+        <TextBlock Text="{Binding DialogTitle}" Classes="dialogtitle"/>
 
         <Grid ColumnDefinitions="*,10,*">
           <StackPanel Grid.Column="0" Spacing="4">

LehrerApp.Desktop/Views/Students/StudentDetailView.axaml

@@ -1,6 +1,7 @@
 <UserControl xmlns="https://github.com/avaloniaui"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              xmlns:vm="clr-namespace:LehrerApp.Desktop.ViewModels.Students"
+             xmlns:shared="clr-namespace:LehrerApp.Desktop.Views.Shared"
              x:Class="LehrerApp.Desktop.Views.Students.StudentDetailView"
              x:DataType="vm:StudentDetailViewModel">
   <Grid RowDefinitions="Auto,*">
@@ -10,9 +11,7 @@
             BorderBrush="{DynamicResource SystemControlForegroundBaseLowBrush}"
             BorderThickness="0,0,0,1">
       <Grid ColumnDefinitions="*,Auto">
-        <StackPanel Grid.Column="0"  IsVisible="{Binding !IsEditing}">
-          <TextBlock Text="{Binding StudentTitle}" FontSize="22" FontWeight="SemiBold"/>
-        </StackPanel>
+        <shared:PageHeader Grid.Column="0" IsVisible="{Binding !IsEditing}" Title="{Binding StudentTitle}"/>
         <StackPanel Grid.Column="0" Spacing="6" IsVisible="{Binding IsEditing}">
           <Grid ColumnDefinitions="*,8,*">
             <TextBox Grid.Column="0" Text="{Binding EditFirstName}" PlaceholderText="Vorname"/>
@@ -181,7 +180,7 @@
               </ItemsControl.ItemTemplate>
             </ItemsControl>
 
-            <TextBlock Text="Noch keine Noten für diesen Schüler erfasst." Opacity="0.4"
+            <TextBlock Text="Noch keine Noten für diesen Schüler erfasst." Classes="emptyhint"
                        IsVisible="{Binding !GradeHistory.Count}"/>
           </StackPanel>
         </ScrollViewer>

LehrerApp.Desktop/Views/Students/StudentListView.axaml

@@ -1,6 +1,7 @@
 <UserControl xmlns="https://github.com/avaloniaui"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              xmlns:vm="clr-namespace:LehrerApp.Desktop.ViewModels.Students"
+             xmlns:shared="clr-namespace:LehrerApp.Desktop.Views.Shared"
              x:Class="LehrerApp.Desktop.Views.Students.StudentListView"
              x:DataType="vm:StudentListViewModel">
   <Grid RowDefinitions="Auto,*">
@@ -8,13 +9,7 @@
             BorderBrush="{DynamicResource SystemControlForegroundBaseLowBrush}"
             BorderThickness="0,0,0,1">
       <Grid ColumnDefinitions="*,Auto,Auto">
-        <StackPanel Grid.Column="0">
-          <TextBlock Text="Schüler" FontSize="22" FontWeight="SemiBold"/>
-          <TextBlock FontSize="12" Opacity="0.5">
-            <Run Text="{Binding Students.Count}"/>
-            <Run Text=" Schüler gesamt"/>
-          </TextBlock>
-        </StackPanel>
+        <shared:PageHeader Grid.Column="0" Title="Schüler" Subtitle="{Binding CountSummary}"/>
         <CheckBox Grid.Column="1" Content="Inaktive anzeigen"
                   IsChecked="{Binding ShowInactive}"
                   VerticalAlignment="Center" Margin="0,0,12,0"/>

TODO.md

@@ -580,12 +580,33 @@ Fächer- und Kompetenzverwaltung existiert bereits in
       sinnvoll zu testen.
 
 ### 13.4 Codepflege
-- [ ] **13.4.1** `CLAUDE.md` mit Projektkonventionen anlegen (`/init`).
+- [x] **13.4.1** `CLAUDE.md` mit Projektkonventionen anlegen (`/init`) — [CLAUDE.md](CLAUDE.md).
 - [ ] **13.4.2** `AllRepositories.cs` und `IRepositories.cs` in Themendateien aufteilen,
-      sobald weitere Repositories dazukommen.
-- [ ] **13.4.3** Wiederkehrende UI-Muster (Suchleiste, Kopfzeile, leere Liste) als
-      wiederverwendbare Controls in `Views/Shared/`.
-- [ ] **13.4.4** Konverter und Styles zentralisieren (aktuell teils inline in den Views).
+      sobald weitere Repositories dazukommen. Aktuell (319 bzw. 154 Zeilen für ~20 Repositories)
+      noch nicht unübersichtlich genug, um die Bedingung auszulösen — bewusst zurückgestellt.
+- [x] **13.4.3** Wiederkehrende UI-Muster als wiederverwendbare Controls/Styles.
+      Bestandsaufnahme vor der Umsetzung ergab drei echte Duplikate (Suchleiste dagegen nur 3×
+      und jedes Mal eine simple `TextBox` ohne gemeinsames Chrome — Extraktion lohnt sich dort
+      nicht):
+      - Seiten-Kopfzeile (Titel 22px SemiBold + optionaler Untertitel) war identisch in
+        `GroupListView`, `StudentListView`, `GroupDetailView`, `StudentDetailView`,
+        `SettingsView` nachgebaut → neue
+        [PageHeader](LehrerApp.Desktop/Views/Shared/PageHeader.axaml)-Control, dort eingesetzt.
+      - Dialog-Titel (`FontSize="18" FontWeight="SemiBold"` als erstes Element) kam in 16
+        Dialogen jeweils inline vor → globale Style-Klasse `TextBlock.dialogtitle`
+        ([App.axaml](LehrerApp.Desktop/App.axaml)).
+      - Leerlisten-Hinweis ("Noch keine …") kam an 10 Stellen vor, dabei war Opacity/FontSize
+        bereits leicht auseinandergedriftet (0.35 vs. 0.4, 12 vs. 13) → globale Style-Klasse
+        `TextBlock.emptyhint`, normalisiert alle Stellen auf einen Wert.
+      Absichtlich nicht angefasst: die vier Views mit eigenen lokalen `<Style Selector=>`-Blöcken
+      (MainWindow, DashboardView, StudentDetailView, ExamEvaluationDialog) sind jeweils
+      einzigartige Spezial-Visualisierungen (Kalenderzellen, Sparkline, Balkendiagramm,
+      Toast/Drawer) ohne Duplikate untereinander.
+- [x] **13.4.4** Konverter und Styles zentralisieren. Bei der Bestandsaufnahme für 13.4.3 zeigte
+      sich: es gibt keine einzige eigene `IValueConverter`-Implementierung im Projekt — überall
+      werden bereits konsistent Avalonias eingebaute statische Konverter (`StringConverters`,
+      `BoolConverters`, `ObjectConverters`) verwendet. Nichts zu zentralisieren; die
+      Style-Duplikate wurden im Zuge von 13.4.3 behoben (siehe oben).
 
 ---