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)