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/ai/SessionList.tsx

@@ -347,7 +347,7 @@ export function SessionList({ activeSessionId, onSelectSession, isSidebar, class
 
   return (
     <div className={clsx('flex flex-col h-full', className)}>
-      {error && <div className="p-2 text-xs text-red-600">{error}</div>}
+      {error && <div className="p-2 text-xs text-red-600">{typeof error === "string" ? error : (error as any)?.message || "Ein Fehler ist aufgetreten"}</div>}
       <div
         className="flex-1 overflow-y-auto p-2"
         onContextMenu={(e) => {