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/wiki/WikiBrowser.tsx

@@ -180,7 +180,7 @@ export function WikiBrowser({ selectedArticleId, onSelectArticle }: WikiBrowserP
             <Loader2 className="animate-spin h-6 w-6 text-primary-500" aria-hidden="true" />
           </div>
         )}
-        {!loading && error && <p className="text-sm text-danger-600 p-3">{error}</p>}
+        {!loading && error && <p className="text-sm text-danger-600 p-3">{typeof error === "string" ? error : (error as any)?.message || "Ein Fehler ist aufgetreten"}</p>}
         {!loading && !error && (
           <>
             <button