Fix update modal hanging indefinitely after completion (related to #628)

When updates complete quickly, the status API may return 'completed' before
the frontend detects the 'restarting' phase. This left users staring at a
frozen modal with no feedback, requiring manual page refresh.

Changes:
- When status is 'completed', immediately check /api/health
- If backend is healthy, reload the page to get new version
- If health check fails, assume restart in progress and start health polling
- Ensures users always get reloaded to the new version automatically

This fixes the UX issue reported in discussion #628 where the update modal
appeared frozen indefinitely despite successful update completion.

frontend-modern/src/components/UpdateProgressModal.tsx

@@ -49,6 +49,28 @@ export function UpdateProgressModal(props: UpdateProgressModalProps) {
         currentStatus.status === 'idle' ||
         currentStatus.status === 'error'
       ) {
+        // If completed successfully, verify backend health and reload to get new version
+        if (currentStatus.status === 'completed' && !currentStatus.error) {
+          if (pollInterval) {
+            clearInterval(pollInterval);
+          }
+          // Verify backend is healthy and reload
+          try {
+            const healthCheck = await fetch('/api/health', { cache: 'no-store' });
+            if (healthCheck.ok) {
+              logger.info('Update completed, backend healthy, reloading...');
+              window.location.reload();
+              return;
+            }
+          } catch (error) {
+            logger.warn('Update completed but health check failed, assuming restart...', error);
+          }
+          // If health check failed, assume restart in progress
+          setIsRestarting(true);
+          startHealthCheckPolling();
+          return;
+        }
+
         setIsComplete(true);
         if (currentStatus.status === 'error' || currentStatus.error) {
           setHasError(true);