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/knowledge/KnowledgeGraph.tsx

@@ -236,7 +236,7 @@ export function KnowledgeGraph() {
       )}
       {!loading && error && (
         <div className="py-8 text-center">
-          <p className="text-danger-600 text-sm mb-4">{error}</p>
+          <p className="text-danger-600 text-sm mb-4">{typeof error === "string" ? error : (error as any)?.message || "Ein Fehler ist aufgetreten"}</p>
           <Button variant="secondary" onClick={() => void load()}>
             {t('common.retry')}
           </Button>