Restructure step 5: one media pipeline on a player host

- PlayerHost port (MediaKitHost): players by role, pins, hwdec policy,
  render probe, log routing, disposal order; the only media_kit user.
- MediaPipeline: plan (one pure switch over state, origin, caps,
  presence) -> resolve -> open (prepared player promoted, else fresh)
  -> ready; commands queued during a switch apply once (P15); viewers
  play the buffer and wait for a dropped source without seek loops (D6).
- Every web page resolved by our yt-dlp into an EDL with headers and
  cookies (danmaku excluded); mpv's ytdl_hook no longer used (ytdl=no).
- D11: sign-in errors say which cookies were used and whether they hold
  any for the site.
- Relay: 256 KiB slices read as they fill, superseded pulls released
  (seek-to-play ~4.2 s -> ~2.8 s, P29); own Jellyfin relay channel only
  when someone needs it (P28).
- STARSYNC_SMOKE run and a Windows CI smoke step after the build.

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

apps/desktop/lib/src/shell/session_player.dart

@@ -11,6 +11,7 @@ import '../controls/controls_probe.dart';
 import '../controls/model/controls_model.dart';
 import '../controls/session_controls.dart';
 import '../controls/volume.dart';
+import '../platform/desktop/media_kit_host.dart' show videoControllerOf;
 import '../queue/queue_ui.dart';
 import '../playback/media_marks.dart' show SkipKind, SkipOffer, formatClock;
 import '../screens/session/jellyfin_widgets.dart';
@@ -321,7 +322,7 @@ class _SessionPlayerState extends State<SessionPlayer> {
 
   Widget _video(VideoControlsBuilder? controls) => Video(
         key: _videoKey,
-        controller: _coord.playback.video,
+        controller: videoControllerOf(_coord.playback.video),
         subtitleViewConfiguration: const SubtitleViewConfiguration(visible: false),
         controls: controls,
       );
@@ -653,7 +654,7 @@ class _SessionPlayerState extends State<SessionPlayer> {
                         ],
                       );
                     }),
-                    _MiniSeekBar(onSeek: (d) => playback.player.seek(d)),
+                    _MiniSeekBar(onSeek: playback.userSeek),
                   ],
                 ),
               ),