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/AskKnowledge.tsx

@@ -90,7 +90,7 @@ export function AskKnowledge() {
           })}
         </div>
 
-        {error && <p className="text-sm text-danger-600" role="alert">{error}</p>}
+        {error && <p className="text-sm text-danger-600" role="alert">{typeof error === "string" ? error : (error as any)?.message || "Ein Fehler ist aufgetreten"}</p>}
 
         {loading && (
           <div className="flex items-center justify-center py-10" role="status" aria-label={t('common.loading')}>

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>