[client] Report the first paint from unstamped windows too

The main and settings windows carry no generation token, so ReadySignal
saw an empty generation that already matched the ref's initial value and
never emitted the painted event. Those windows only became visible through
the fallback timer, and their frontend was never marked mounted, so the
login trigger and the requested settings tab stayed queued.

Start the ref from null so the first report goes out regardless of the
generation value.

client/ui/frontend/src/components/ReadySignal.tsx

@@ -9,7 +9,7 @@ export const ReadySignal = () => {
     const { isReady } = useStatus();
     const [params] = useSearchParams();
     const generation = params.get("gen") ?? "";
-    const sent = useRef("");
+    const sent = useRef<string | null>(null);
 
     useEffect(() => {
         if (!isReady || sent.current === generation) return;