Add constant rich text placeholders

LehrerApp.Templating/TemplateDataResolver.cs

@@ -0,0 +1,73 @@
+using System.Globalization;
+
+namespace LehrerApp.Templating;
+
+public static class TemplateDataResolver
+{
+    public static IReadOnlyList<string> ValidateConstants(TemplateManifest manifest)
+    {
+        var issues = new List<string>();
+        var definitions = manifest.Placeholders.GroupBy(x => x.Name, StringComparer.Ordinal)
+            .ToDictionary(x => x.Key, x => x.First(), StringComparer.Ordinal);
+        foreach (var constant in manifest.Placeholders.Where(x => x.IsConstant))
+        {
+            try
+            {
+                ConstantValue(constant);
+                if (constant.Type is not (PlaceholderType.Text or PlaceholderType.Multiline)) continue;
+                foreach (var reference in TemplateRichText.UsedPlaceholders(constant.ConstantValue ?? ""))
+                {
+                    if (!definitions.TryGetValue(reference, out var target))
+                        issues.Add($"Konstanter Text „{constant.Name}“ verwendet den nicht deklarierten Platzhalter „{reference}“.");
+                    else if (target.IsConstant)
+                        issues.Add($"Konstanter Text „{constant.Name}“ darf nur externe Platzhalter verwenden; „{reference}“ ist ebenfalls konstant.");
+                    else if (target.Type is not (PlaceholderType.Text or PlaceholderType.Multiline
+                                 or PlaceholderType.Date or PlaceholderType.Number))
+                        issues.Add($"Eingebetteter Platzhalter „{reference}“ hat keinen textuell darstellbaren Typ.");
+                }
+            }
+            catch (InvalidDataException ex) { issues.Add(ex.Message); }
+        }
+        return issues;
+    }
+
+    public static IReadOnlyDictionary<string, PlaceholderValue> Resolve(TemplateManifest manifest,
+        IReadOnlyDictionary<string, PlaceholderValue> externalValues)
+    {
+        var result = new Dictionary<string, PlaceholderValue>(externalValues, StringComparer.Ordinal);
+        foreach (var definition in manifest.Placeholders.Where(x => x.IsConstant))
+            result[definition.Name] = ConstantValue(definition);
+        return result;
+    }
+
+    public static PlaceholderValue ConstantValue(PlaceholderDefinition definition)
+    {
+        var raw = definition.ConstantValue ?? "";
+        return definition.Type switch
+        {
+            PlaceholderType.Text => new TextValue(raw),
+            PlaceholderType.Multiline => new MultilineValue(raw),
+            PlaceholderType.Date => new DateValue(ParseDate(definition, raw)),
+            PlaceholderType.Number => new NumberValue(ParseNumber(definition, raw)),
+            _ => throw new InvalidDataException(
+                $"Platzhalter „{definition.Name}“ vom Typ {definition.Type} kann keinen konstanten Textwert verwenden."),
+        };
+    }
+
+    private static DateOnly ParseDate(PlaceholderDefinition definition, string raw)
+    {
+        if (DateOnly.TryParse(raw, CultureInfo.InvariantCulture, DateTimeStyles.None, out var date)
+            || DateOnly.TryParse(raw, CultureInfo.GetCultureInfo("de-DE"), DateTimeStyles.None, out date)) return date;
+        throw Error(definition, "Datum, z. B. 2026-08-30");
+    }
+
+    private static decimal ParseNumber(PlaceholderDefinition definition, string raw)
+    {
+        if (decimal.TryParse(raw, NumberStyles.Number, CultureInfo.InvariantCulture, out var number)
+            || decimal.TryParse(raw, NumberStyles.Number, CultureInfo.GetCultureInfo("de-DE"), out number)) return number;
+        throw Error(definition, "Zahl");
+    }
+
+    private static InvalidDataException Error(PlaceholderDefinition definition, string expected) =>
+        new($"Konstanter Wert für „{definition.Name}“ ist ungültig; erwartet wird: {expected}.");
+}