fix(frontend): stop SyncModal polling once sync has finished

It polled /api/sync/status every 1.5s indefinitely even after
in_progress became false and the final result was already shown,
only stopping on unmount (clicking Close). Skip scheduling the next
poll once a fetched status reports the sync is no longer running.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/SyncModal.tsx

@@ -34,9 +34,14 @@ export function SyncModal({ onClose }: { onClose: () => void }) {
         .then((s) => {
           setStatus(s);
           setError(null);
+          // Once a fetched status reports the sync finished, stop polling --
+          // the final result is already shown, and there's nothing new left
+          // to fetch until the user starts another sync ("Close" unmounts
+          // this component, which is the only other way polling stops).
+          if (!stoppedRef.current && s.in_progress) timeout = setTimeout(tick, 1500);
         })
-        .catch((e) => setError(String(e)))
-        .finally(() => {
+        .catch((e) => {
+          setError(String(e));
           if (!stoppedRef.current) timeout = setTimeout(tick, 1500);
         });
     };