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/components/window/AiChatPanel.tsx

@@ -135,7 +135,7 @@ export function AiChatPanel({ windowTitle, windowType }: AiChatPanelProps) {
         <div className="flex-1 flex items-center justify-center p-4 text-center">
           <div>
             <Sparkles className="w-8 h-8 text-secondary-300 mx-auto mb-2" />
-            <p className="text-sm text-secondary-500">{error}</p>
+            <p className="text-sm text-secondary-500">{typeof error === "string" ? error : (error as any)?.message || "Ein Fehler ist aufgetreten"}</p>
           </div>
         </div>
       </div>
@@ -180,7 +180,7 @@ export function AiChatPanel({ windowTitle, windowType }: AiChatPanelProps) {
           </div>
         )}
         {error && (
-          <div className="text-xs text-danger-600 px-2 py-1">{error}</div>
+          <div className="text-xs text-danger-600 px-2 py-1">{typeof error === "string" ? error : (error as any)?.message || "Ein Fehler ist aufgetreten"}</div>
         )}
       </div>