fix: prevent [object Object] rendering in 25 frontend components

Replace direct {error} JSX rendering with typeof check + .message fallback.
When error is an object (not a string), React showed [object Object].
Now renders error.message or fallback string.

frontend/src/pages/Communication.tsx

@@ -501,7 +501,7 @@ function ChatWindow({ convId, category, onBack }: ChatWindowProps) {
           </div>
         )}
         {error && (
-          <div className="text-sm text-red-600 bg-red-50 rounded p-3">{error}</div>
+          <div className="text-sm text-red-600 bg-red-50 rounded p-3">{typeof error === "string" ? error : (error as any)?.message || "Ein Fehler ist aufgetreten"}</div>
         )}
         {!loading && messages.length === 0 && !aiStreaming && (
           <div className="text-center text-secondary-400 py-8">