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- Relay: 256 KiB slices read as they fill, superseded pulls released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/main.dart

@@ -3,15 +3,16 @@ import 'dart:io';
 
 import 'package:flutter/material.dart';
 import 'package:hive_ce_flutter/hive_flutter.dart';
-import 'package:media_kit/media_kit.dart';
 import 'package:sync_relay_core/core.dart';
 
 import 'app.dart';
 import 'src/app/app_graph.dart';
+import 'src/files/thumbnails.dart' show thumbnailPlayers;
 import 'src/platform/app_log.dart';
 import 'src/platform/app_shutdown.dart' show closeAppWith;
 import 'src/platform/desktop/desktop_app_dirs.dart';
 import 'src/platform/desktop_entry.dart';
+import 'src/platform/smoke.dart';
 import 'src/screens/home/ui.dart';
 import 'src/session/app_events.dart' show UpdateAvailable;
 import 'src/state/settings.dart';
@@ -67,7 +68,8 @@ Future<void> main(List<String> args) async {
   }
   // The library's matches, pins and offsets (device-owned).
   unawaited(graph.library.ensureLoaded());
-  MediaKit.ensureInitialized();
+  ports.players.init();
+  thumbnailPlayers = ports.players;
   // yt-dlp for web page links: downloaded on first run, kept current.
   ports.links.warmUp();
   // Linux: the launcher entry and icon Wayland needs (release installs).
@@ -82,6 +84,11 @@ Future<void> main(List<String> args) async {
     }
   });
   runApp(AppScope(graph: graph, child: const WatchTogetherApp()));
+  // CI's close smoke run: open a file, play, close (see smoke.dart).
+  final smoke = smokeMedia;
+  if (smoke != null) {
+    unawaited(WidgetsBinding.instance.endOfFrame.then((_) => runSmoke(graph, smoke)));
+  }
   // Home: your own server, quietly (only this app can reach it until
   // Invite).
   unawaited(graph.navigation.start());