fix: prevent crash on login when user has no active threads

activeThreads[0].id in useEffect dependency array threw when the array
was empty. Use optional chaining to safely handle the empty case.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/client/components/AddToThreadModal.tsx

@@ -63,7 +63,7 @@ export function AddToThreadModal() {
 		open,
 		activeThreads.length,
 		activeThreads.some,
-		activeThreads[0].id,
+		activeThreads[0]?.id,
 		catalogSessionThreadId,
 	]);