Chapters on the scrubber, Skip OP/ED, audio/subtitle menus

- PlaybackController exposes chapters, segments (Jellyfin
  MediaSegments for account holders), tracks, position/duration.
- Own seek bar with chapter diamonds and chapter title on hover,
  seeking through the sync user-seek path.
- Skip OP / Skip ED pills from Intro/Outro segments or chapter names
  (ED skips to its end only); Skip 1:30 fallback for >20 min videos
  without OP info; next-episode pill now starts at the ED.
- PgUp/PgDn chapters, Shift+Right +1:30; Audio and Subtitles menus
  with count badges via the existing track path.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/test/jellyfin_e2e_test.dart

@@ -9,6 +9,7 @@ import 'package:sync_relay_server/server.dart';
 import 'package:watch_together/src/jellyfin/accounts.dart';
 import 'package:watch_together/src/jellyfin/session_media.dart';
 import 'package:watch_together/src/jellyfin/track_prefs.dart';
+import 'package:watch_together/src/playback/media_marks.dart';
 import 'package:watch_together/src/session/session_coordinator.dart';
 
 // Two account holders and a guest watch Jellyfin episodes together over a
@@ -134,6 +135,15 @@ class _FakeJellyfin {
     if (path == '/Shows/$_series/Episodes') {
       return json({'Items': [_item(_ep1), _item(_ep2)], 'TotalRecordCount': 2});
     }
+    if (path == '/MediaSegments/$_ep1') {
+      return json({
+        'Items': [
+          {'Id': 'seg1', 'ItemId': _ep1, 'Type': 'Intro', 'StartTicks': 20000000, 'EndTicks': 60000000},
+        ],
+        'TotalRecordCount': 1,
+        'StartIndex': 0,
+      });
+    }
     if (path.startsWith('/Sessions/Playing')) {
       res.statusCode = 204;
       return res.close();
@@ -260,6 +270,13 @@ void main() {
         }, 'jf access in the member list');
         expect(a.nextEpisode.peek()?.item.id, _ep2);
         expect(b.nextEpisode.peek(), isNull, reason: 'guests cannot resolve the next one');
+        // MediaSegments with an account; the guest (relay) has none.
+        await _poll(() => a.playback.segments.peek().isNotEmpty, 'segments');
+        expect(a.playback.opening.peek()?.source, SkipSource.segment);
+        expect(a.playback.opening.peek()?.end, const Duration(seconds: 6));
+        expect(b.playback.segments.peek(), isEmpty);
+        expect(jelly.calls.where((c) => c.path.startsWith('/MediaSegments') && _FakeJellyfin.authFields(c.auth)['Token'] == 'CTOKEN'),
+            isNotEmpty);
 
         for (final x in all) {
           x.toggleReady();