fix: Dashboard-Kachelraster - Luecken und falsche Margins bei ausgeblendeten Kacheln

ApplyCardLayout() zaehlte bisher IsVisible statt EffectiveIsVisible, wodurch eine
eingeschaltete aber leere HideWhenEmpty-Kachel weiterhin einen Rasterplatz belegte.
Da im Alltag meist mehrere der sieben HideWhenEmpty-Kacheln leer sind, war das der
Normalfall, nicht die Ausnahme. UpdateDashboardSummary() setzte zudem alle IsEmpty-
Werte, ohne das Layout danach neu zu berechnen.

Zusaetzlich hatte jede Kachel ihren Grid-Margin fest im XAML verdrahtet (links/rechts),
obwohl Spalte und Zeile erst zur Laufzeit aus Sichtbarkeit und Reihenfolge berechnet
werden - beim Ausblenden einer Kachel wanderten die Nachbarn in die andere Spalte,
der Rinnstein sass dann auf der falschen Seite. DashboardCardOption.Margin leitet den
Wert jetzt aus Column ab; das XAML bindet darauf statt fixer Werte.

Drei Regressionstests decken beide Faelle ab.

LehrerApp.Desktop/Views/Dashboard/DashboardView.axaml

@@ -146,7 +146,7 @@
 
         <!-- Heutige Stunden -->
         <Border Grid.Column="{Binding TodayCard.Column}" Grid.Row="{Binding TodayCard.Row}"
-                IsVisible="{Binding TodayCard.EffectiveIsVisible}" Margin="0,0,8,8"
+                IsVisible="{Binding TodayCard.EffectiveIsVisible}" Margin="{Binding TodayCard.Margin}"
                 Background="{DynamicResource SystemControlBackgroundAltHighBrush}"
                 CornerRadius="8" Padding="16">
           <StackPanel>
@@ -187,7 +187,7 @@
 
         <!-- Offene Aufgaben -->
         <Border Grid.Column="{Binding TasksCard.Column}" Grid.Row="{Binding TasksCard.Row}"
-                IsVisible="{Binding TasksCard.EffectiveIsVisible}" Margin="8,0,0,8"
+                IsVisible="{Binding TasksCard.EffectiveIsVisible}" Margin="{Binding TasksCard.Margin}"
                 Background="{DynamicResource SystemControlBackgroundAltHighBrush}"
                 CornerRadius="8" Padding="16">
           <StackPanel>
@@ -224,7 +224,7 @@
         <!-- Kalender: feste Position direkt unter Heute/Aufgaben, damit die wachsende
              Lerngruppen-Liste darunter ihn nicht nach unten verdrängt. -->
         <Border Grid.Column="{Binding CalendarCard.Column}" Grid.Row="{Binding CalendarCard.Row}"
-                IsVisible="{Binding CalendarCard.EffectiveIsVisible}" Margin="0,0,8,8"
+                IsVisible="{Binding CalendarCard.EffectiveIsVisible}" Margin="{Binding CalendarCard.Margin}"
                 Background="{DynamicResource SystemControlBackgroundAltHighBrush}"
                 CornerRadius="8" Padding="16" HorizontalAlignment="Left" MaxWidth="320">
           <StackPanel Spacing="8">
@@ -368,7 +368,7 @@
 
         <!-- Offene Entschuldigungen: neben dem Kalender, ebenfalls feste Position -->
         <Border Grid.Column="{Binding ExcusesCard.Column}" Grid.Row="{Binding ExcusesCard.Row}"
-                IsVisible="{Binding ExcusesCard.EffectiveIsVisible}" Margin="8,0,0,8" VerticalAlignment="Top"
+                IsVisible="{Binding ExcusesCard.EffectiveIsVisible}" Margin="{Binding ExcusesCard.Margin}" VerticalAlignment="Top"
                 Background="{DynamicResource SystemControlBackgroundAltHighBrush}"
                 CornerRadius="8" Padding="16">
           <StackPanel>
@@ -402,7 +402,7 @@
 
         <!-- Fehlzeiten-Warnung (5.2.3) -->
         <Border Grid.Column="{Binding AttendanceCard.Column}" Grid.Row="{Binding AttendanceCard.Row}"
-                IsVisible="{Binding AttendanceCard.EffectiveIsVisible}" Margin="0,0,8,8" VerticalAlignment="Top"
+                IsVisible="{Binding AttendanceCard.EffectiveIsVisible}" Margin="{Binding AttendanceCard.Margin}" VerticalAlignment="Top"
                 Background="{DynamicResource SystemControlBackgroundAltHighBrush}"
                 CornerRadius="8" Padding="16">
           <StackPanel>
@@ -432,7 +432,7 @@
 
         <!-- Klausurwochen (Nutzer-Feedback): eigene Klausurlast über alle Kurse hinweg -->
         <Border Grid.Column="{Binding ExamLoadCard.Column}" Grid.Row="{Binding ExamLoadCard.Row}"
-                IsVisible="{Binding ExamLoadCard.EffectiveIsVisible}" Margin="0,0,8,8" VerticalAlignment="Top"
+                IsVisible="{Binding ExamLoadCard.EffectiveIsVisible}" Margin="{Binding ExamLoadCard.Margin}" VerticalAlignment="Top"
                 Background="{DynamicResource SystemControlBackgroundAltHighBrush}"
                 CornerRadius="8" Padding="16">
           <StackPanel>
@@ -456,7 +456,7 @@
 
         <!-- Unterrichtszeit nacherfassen (Nutzer-Feedback) -->
         <Border Grid.Column="{Binding MissingTeachingTimeCard.Column}" Grid.Row="{Binding MissingTeachingTimeCard.Row}"
-                IsVisible="{Binding MissingTeachingTimeCard.EffectiveIsVisible}" Margin="0,0,8,8" VerticalAlignment="Top"
+                IsVisible="{Binding MissingTeachingTimeCard.EffectiveIsVisible}" Margin="{Binding MissingTeachingTimeCard.Margin}" VerticalAlignment="Top"
                 Background="{DynamicResource SystemControlBackgroundAltHighBrush}"
                 CornerRadius="8" Padding="16">
           <StackPanel>
@@ -482,7 +482,7 @@
 
         <!-- Förderplan-Wiedervorlage (5.3.2) -->
         <Border Grid.Column="{Binding SupportCard.Column}" Grid.Row="{Binding SupportCard.Row}"
-                IsVisible="{Binding SupportCard.EffectiveIsVisible}" Margin="8,0,0,8" VerticalAlignment="Top"
+                IsVisible="{Binding SupportCard.EffectiveIsVisible}" Margin="{Binding SupportCard.Margin}" VerticalAlignment="Top"
                 Background="{DynamicResource SystemControlBackgroundAltHighBrush}"
                 CornerRadius="8" Padding="16">
           <StackPanel>
@@ -513,7 +513,7 @@
 
         <!-- Anstehende Termine (9.3) -->
         <Border Grid.Column="{Binding UpcomingCard.Column}" Grid.Row="{Binding UpcomingCard.Row}"
-                IsVisible="{Binding UpcomingCard.EffectiveIsVisible}" Margin="0,0,8,8" VerticalAlignment="Top"
+                IsVisible="{Binding UpcomingCard.EffectiveIsVisible}" Margin="{Binding UpcomingCard.Margin}" VerticalAlignment="Top"
                 Background="{DynamicResource SystemControlBackgroundAltHighBrush}"
                 CornerRadius="8" Padding="16">
           <StackPanel>
@@ -550,7 +550,7 @@
 
         <!-- Offene Korrekturen (9.4) -->
         <Border Grid.Column="{Binding CorrectionsCard.Column}" Grid.Row="{Binding CorrectionsCard.Row}"
-                IsVisible="{Binding CorrectionsCard.EffectiveIsVisible}" Margin="8,0,0,8" VerticalAlignment="Top"
+                IsVisible="{Binding CorrectionsCard.EffectiveIsVisible}" Margin="{Binding CorrectionsCard.Margin}" VerticalAlignment="Top"
                 Background="{DynamicResource SystemControlBackgroundAltHighBrush}"
                 CornerRadius="8" Padding="16">
           <StackPanel>
@@ -583,7 +583,7 @@
 
         <!-- Ungeplante Stunden -->
         <Border Grid.Column="{Binding UnplannedCard.Column}" Grid.Row="{Binding UnplannedCard.Row}"
-                IsVisible="{Binding UnplannedCard.EffectiveIsVisible}" Margin="0,0,8,8" VerticalAlignment="Top"
+                IsVisible="{Binding UnplannedCard.EffectiveIsVisible}" Margin="{Binding UnplannedCard.Margin}" VerticalAlignment="Top"
                 Background="{DynamicResource SystemControlBackgroundAltHighBrush}"
                 CornerRadius="8" Padding="16">
           <StackPanel>
@@ -613,7 +613,7 @@
 
         <!-- Auffälligkeiten (9.5) -->
         <Border Grid.Column="{Binding AlertsCard.Column}" Grid.Row="{Binding AlertsCard.Row}"
-                IsVisible="{Binding AlertsCard.EffectiveIsVisible}" Margin="0,0,8,8" VerticalAlignment="Top"
+                IsVisible="{Binding AlertsCard.EffectiveIsVisible}" Margin="{Binding AlertsCard.Margin}" VerticalAlignment="Top"
                 Background="{DynamicResource SystemControlBackgroundAltHighBrush}"
                 CornerRadius="8" Padding="16">
           <StackPanel>
@@ -647,7 +647,7 @@
 
         <!-- Meine Lerngruppen -->
         <Border Grid.Column="{Binding GroupsCard.Column}" Grid.Row="{Binding GroupsCard.Row}"
-                IsVisible="{Binding GroupsCard.EffectiveIsVisible}"
+                IsVisible="{Binding GroupsCard.EffectiveIsVisible}" Margin="{Binding GroupsCard.Margin}"
                 Background="{DynamicResource SystemControlBackgroundAltHighBrush}"
                 CornerRadius="8" Padding="16">
           <StackPanel>