Add constant rich text placeholders

LehrerApp.Templating/TemplateLoader.cs

@@ -111,6 +111,8 @@ public sealed class TemplateLoader(TemplateLimits? limits = null,
                 issues.Add(new(ValidationSeverity.Error, $"Platzhalter „{used}“ wird im Layout verwendet, aber nicht im Manifest deklariert."));
             foreach (var duplicate in manifest.Placeholders.GroupBy(x => x.Name, StringComparer.Ordinal).Where(x => x.Count() > 1))
                 issues.Add(new(ValidationSeverity.Error, $"Platzhalter „{duplicate.Key}“ ist mehrfach deklariert."));
+            foreach (var issue in TemplateDataResolver.ValidateConstants(manifest))
+                issues.Add(new(ValidationSeverity.Error, issue));
             if (issues.Count > 0) throw new TemplateValidationException(new(issues));
             return new(manifest, layout, assets, sourceName);
         }
@@ -123,6 +125,7 @@ public sealed class TemplateLoader(TemplateLimits? limits = null,
         var issues = new List<ValidationIssue>();
         foreach (var placeholder in template.Manifest.Placeholders)
         {
+            if (placeholder.IsConstant) continue;
             if (!providedTypes.TryGetValue(placeholder.Name, out var actual))
             {
                 if (placeholder.Required) issues.Add(new(ValidationSeverity.Error, $"Pflichtwert „{placeholder.Name}“ fehlt."));