feat(plex): multi-rendition audio (client-side switch) + reliable resume-on-F5

- Multi-audio items now ship every audio track as an HLS rendition in one session
  (stream.py var_stream_map -> master.m3u8), so hls.js switches audio CLIENT-SIDE with
  no ffmpeg restart, same timeline, no gap/drift. /session gains ?multi=1 (forces HLS
  even for direct-playable multi-audio files); K is probed from the video variant seg.
- Restore the selected audio track on AUDIO_TRACKS_UPDATED, not MANIFEST_PARSED (the
  renditions aren't parsed yet on MANIFEST_PARSED, so the set was dropped -> after F5 the
  UI showed the restored track but playback stayed on the default).
- Resume position now survives F5: a pagehide keepalive beacon (plexProgressBeacon)
  saves the current position on reload/close/navigate, since React effect cleanup does
  not run on a full reload; seekTo writes the target into absRef immediately so a save
  right after a seek is accurate.

frontend/src/components/PlexPlayer.tsx

@@ -133,6 +133,10 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
   const wrapRef = useRef<HTMLDivElement>(null);
   const sessionStartRef = useRef(0); // absolute offset the current HLS/direct session begins at
   const durationRef = useRef(0);
+  // ≥2 audio tracks → multi-rendition HLS: all tracks ship in one session and hls.js switches audio
+  // client-side (no restart). Ref so the stable loadSession/changeAudio callbacks read it fresh.
+  const multiAudioRef = useRef(false);
+  multiAudioRef.current = (detail?.audio_streams.length ?? 0) > 1;
 
   const [playing, setPlaying] = useState(false);
   const [abs, setAbs] = useState(0); // absolute current time
@@ -213,7 +217,10 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
       setLoadError(null);
       let sess;
       try {
-        sess = await api.plexSession(id, startAt, audioRef.current, aoffRef.current);
+        // Multi-audio → request the multi-rendition session (no single `audio` ordinal); the track is
+        // selected client-side after the manifest loads.
+        const m = multiAudioRef.current;
+        sess = await api.plexSession(id, startAt, m ? null : audioRef.current, aoffRef.current, m);
       } catch (e: any) {
         // Surface WHY playback can't start instead of an eternal spinner. 404 = the physical file
         // isn't reachable (missing media bind-mount or a wrong plex_path_map); 501 = the codec
@@ -247,6 +254,21 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
           setReady(true);
           if (aliveRef.current) video.play().catch(() => {});
         });
+        // Restore the selected audio track ONCE hls.js has parsed the renditions — doing it on
+        // MANIFEST_PARSED was too early (the track list wasn't populated, so the set was dropped and
+        // playback stayed on the default track while the UI showed the restored selection).
+        hls.on(Hls.Events.AUDIO_TRACKS_UPDATED, () => {
+          if (multiAudioRef.current) {
+            const want = audioRef.current ?? 0;
+            if (hls.audioTrack !== want) {
+              try {
+                hls.audioTrack = want;
+              } catch {
+                /* out of range — ignore */
+              }
+            }
+          }
+        });
         // A fatal HLS error (e.g. the remux session died / the file became unreadable) would
         // otherwise leave the spinner up forever — surface it instead.
         hls.on(Hls.Events.ERROR, (_e, data) => {
@@ -353,8 +375,17 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
         api.plexProgress(id, Math.floor(absRef.current), durationRef.current).catch(() => {});
     };
     const iv = window.setInterval(flush, 10000);
+    // Save on page unload too (F5/close/navigate): React effect cleanup does NOT run on a full reload,
+    // so without this the resume point lags up to 10s — e.g. a seek right before F5 was lost. Uses a
+    // keepalive beacon so the POST survives the unload.
+    const onHide = () => {
+      if (absRef.current > 0 && durationRef.current > 0)
+        api.plexProgressBeacon(id, Math.floor(absRef.current), durationRef.current);
+    };
+    window.addEventListener("pagehide", onHide);
     return () => {
       window.clearInterval(iv);
+      window.removeEventListener("pagehide", onHide);
       flush();
       qc.invalidateQueries({ queryKey: ["plex-browse"] });
       qc.invalidateQueries({ queryKey: ["plex-show"] });
@@ -368,6 +399,7 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
       if (!video) return;
       const dur = durationRef.current || target;
       const T = Math.max(0, Math.min(target, dur - 1));
+      absRef.current = T; // reflect the target immediately so a save/beacon right after a seek is accurate
       const rel = T - sessionStartRef.current;
       if (rel >= 0 && rel <= (video.seekable.length ? video.seekable.end(0) : 0) + 0.5) {
         video.currentTime = rel;
@@ -461,7 +493,16 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
       setAudioOrd(ord);
       // Remember the LANGUAGE (not the ord) so F5 / the next item restores this choice. Menu stays open.
       patchPrefs({ audioLang: ord == null ? "" : (detail?.audio_streams.find((s) => s.ord === ord)?.language ?? "") });
-      loadSession(absRef.current);
+      if (multiAudioRef.current && hlsRef.current) {
+        // Multi-rendition: switch the audio track CLIENT-SIDE — instant, same timeline, no restart.
+        try {
+          hlsRef.current.audioTrack = ord ?? 0;
+        } catch {
+          /* ignore */
+        }
+      } else {
+        loadSession(absRef.current); // single-audio: re-map the track server-side (session restart)
+      }
     },
     [loadSession, detail, patchPrefs],
   );