Prevent catalog title changes during edit

Enforce immutability of CatTitle on updates: backend now rejects title changes with a BadRequest, and frontend disables the title input field when editing.

DbFirst.API/Controllers/CatalogsController.cs

@@ -54,6 +54,10 @@ public class CatalogsController : ControllerBase
         {
             return NotFound();
         }
+        if (!string.Equals(current.CatTitle, dto.CatTitle, StringComparison.OrdinalIgnoreCase))
+        {
+            return BadRequest("Titel kann nicht geändert werden.");
+        }
 
         var updated = await _mediator.Send(new UpdateCatalogCommand(id, dto), cancellationToken);
         if (updated == null)

DbFirst.BlazorWasm/Pages/Catalogs.razor

@@ -24,7 +24,7 @@ else if (!string.IsNullOrWhiteSpace(infoMessage))
         <div class="row g-3">
             <div class="col-md-6">
                 <label class="form-label">Titel</label>
-                <InputText class="form-control" @bind-Value="formModel.CatTitle" />
+                <InputText class="form-control" @bind-Value="formModel.CatTitle" disabled="@isEditing" />
             </div>
             <div class="col-md-6">
                 <label class="form-label">Kennung</label>