fix(header): keep the sync-status count live (focus refetch + background poll)

The header 'N without full history' (and sync state) polled every 30s but only while
the tab was focused, and the global refetchOnWindowFocus is off — so after switching
tabs the count looked frozen until a manual reload. Poll in the background too and
refetch on focus so it tracks the scheduler's progress without F5.

frontend/src/components/SyncStatus.tsx

@@ -20,7 +20,12 @@ export default function SyncStatus({
   const { data } = useQuery({
     queryKey: ["my-status"],
     queryFn: api.myStatus,
+    // Track the scheduler near-real-time: poll even when the tab is backgrounded, and
+    // refresh immediately on tab focus (the global default disables focus refetch), so the
+    // "N without full history" count keeps ticking down without a manual reload.
     refetchInterval: 30_000,
+    refetchIntervalInBackground: true,
+    refetchOnWindowFocus: true,
     staleTime: 25_000,
   });