fix(frontend): defer date and localStorage reads to post-mount to fix hydration mismatches

frontend/src/components/HeroPlayer.tsx

@@ -23,24 +23,30 @@ const lsSet = (k: string, v: string) => {
 };
 
 export function HeroPlayer({ initialStations }: Props) {
+  // ssr-safe defaults; localStorage + Date reads happen post-mount to avoid
+  // hydration mismatches (#418, #423, #425).
   const [stations, setStations] = useState<Station[]>(initialStations);
-  const [selectedId, setSelectedId] = useState<string>(() => {
-    const saved = lsGet('denpa:station');
-    if (saved && initialStations.some((s) => s.id === saved)) return saved;
-    return initialStations[0]?.id ?? '';
-  });
-  const [format, setFormat] = useState<Format>(() => (lsGet('denpa:format') === 'opus' ? 'opus' : 'mp3'));
-  const [vol, setVol] = useState<number>(() => {
-    const v = parseFloat(lsGet('denpa:vol') ?? '0.72');
-    return isNaN(v) ? 0.72 : Math.max(0, Math.min(1, v));
-  });
+  const [selectedId, setSelectedId] = useState<string>(initialStations[0]?.id ?? '');
+  const [format, setFormat] = useState<Format>('mp3');
+  const [vol, setVol] = useState<number>(0.72);
   const [muted, setMuted] = useState(false);
   const [playing, setPlaying] = useState(false);
   const [now, setNow] = useState<NowPlaying | null>(null);
   const [listeners, setListeners] = useState<number | null>(null);
   const [elapsed, setElapsed] = useState(0);
   const [copyState, setCopyState] = useState<'idle' | 'ok'>('idle');
-  const [clockNow, setClockNow] = useState(() => new Date());
+  const [clockText, setClockText] = useState<{ date: string; time: string }>({ date: '', time: '' });
+
+  // restore persisted UI state once on the client
+  useEffect(() => {
+    const savedStation = lsGet('denpa:station');
+    if (savedStation && initialStations.some((s) => s.id === savedStation)) {
+      setSelectedId(savedStation);
+    }
+    if (lsGet('denpa:format') === 'opus') setFormat('opus');
+    const savedVol = parseFloat(lsGet('denpa:vol') ?? '');
+    if (!isNaN(savedVol)) setVol(Math.max(0, Math.min(1, savedVol)));
+  }, [initialStations]);
 
   const audioRef = useRef<HTMLAudioElement | null>(null);
   const station = stations.find((s) => s.id === selectedId);
@@ -155,9 +161,14 @@ export function HeroPlayer({ initialStations }: Props) {
     return () => clearInterval(id);
   }, [now?.started_at]);
 
-  // wall clock for header
+  // wall clock for header — only ticks after mount so ssr/hydrate match (empty strings)
   useEffect(() => {
-    const id = setInterval(() => setClockNow(new Date()), 1000);
+    const tick = () => {
+      const d = new Date();
+      setClockText({ date: fmtJpDate(d), time: fmtJpTime(d) });
+    };
+    tick();
+    const id = setInterval(tick, 1000);
     return () => clearInterval(id);
   }, []);
 
@@ -238,8 +249,8 @@ export function HeroPlayer({ initialStations }: Props) {
           tune in.<br/>tune out.<br/>melt yr brain {'<3'}
         </div>
         <div className="hero-status">
-          <div>{fmtJpDate(clockNow)}</div>
-          <div>{fmtJpTime(clockNow)}</div>
+          <div>{clockText.date || ' '}</div>
+          <div>{clockText.time || ' '}</div>
           <div className="live">[<span className="blink">{playing ? 'REC' : 'OFF'}</span>] {playing ? 'LIVE' : 'PAUSED'} · {listeners ?? '—'} listening</div>
         </div>
       </div>