fix(plex): mark the finished episode on the correct id when auto-advancing

The player's watched-mark on completion keyed off the reactive `id`, which can run ahead of the
media actually playing: when the next episode fails to load (e.g. no local file), loadSession
returns without replacing the <video> source, so the old episode keeps playing and later fires
'ended' — but by then an auto-skip-credits jump has already advanced `id` to the next episode.
The result was the WRONG episode marked watched (and scrobbled to Plex), while the finished one
got nothing. This was latent before, made visible/harmful by the Phase B Plex push.

- go(): pause the outgoing media before switching, so a failed next-episode load can't leave the
  old media running into a stray 'ended' against the new id.
- auto-skip-credits (doSkip): mark the CURRENT id watched (captured now) before advancing — binge
  advance is a finish, and previously marked nothing.
- onEnded: ignore a spurious 'ended' from media that never played (absRef 0), so a failed next
  episode can't be marked or cascade another advance.

Manual Next/Prev still never mark watched (not a finish). Verified live: E06→E07 with a missing
E07 marks E06 watched (+ Plex), E07 nothing.

frontend/src/components/PlexPlayer.tsx

@@ -566,6 +566,15 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
   const go = useCallback(
     (otherId: string | null | undefined) => {
       if (otherId) {
+        // Stop the outgoing media BEFORE switching. If the next item fails to load (e.g. no local
+        // file), loadSession returns without replacing the <video> source, leaving the old episode
+        // still playing — it would then fire 'ended' against the NEW id and mark the wrong item
+        // watched (and scrobble it to Plex). Pausing here severs that stale-tail race.
+        try {
+          videoRef.current?.pause();
+        } catch {
+          /* ignore */
+        }
         setAbs(0);
         setId(otherId);
       }
@@ -710,6 +719,9 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
     const video = videoRef.current;
     if (!video) return;
     const onEnded = () => {
+      // Only a genuine play-to-the-end counts. A next episode that failed to load never played
+      // (absRef 0) — its stray 'ended' must not mark anything watched or cascade another advance.
+      if (absRef.current <= 0) return;
       api.plexSetState(id, "watched").catch(() => {});
       if (nextId) go(nextId);
     };
@@ -845,10 +857,14 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
   const doSkip = useCallback(
     (m: PlexMarker) => {
       setSkipProgress(null);
-      if (m.type === "credits" && nextId) go(nextId);
+      if (m.type === "credits" && nextId) {
-      else seekTo(m.end_s);
+        // Binge auto-advance: the episode we're leaving was watched through the credits, so mark IT
+        // (the current id, captured here — not the reactive id, which go() is about to change).
+        api.plexSetState(id, "watched").catch(() => {});
+        go(nextId);
+      } else seekTo(m.end_s);
     },
-    [go, nextId, seekTo],
+    [id, go, nextId, seekTo],
   );
   const cancelAutoSkip = useCallback(() => {
     const m = activeMarkerRef.current;