Player: mpv logs at warn (v only while probing the render mode), 4xx fails at once, load errors show

The session player asked libmpv for every v-level message all session and
media_kit drains them on the UI thread: a retry storm could freeze the
window. ffmpeg reconnects only on 5xx, bounded. A load that fails before
a duration is known reports once ('can't play this: HTTP error 403 ...').

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

apps/desktop/lib/src/session/session_coordinator.dart

@@ -333,6 +333,7 @@ class SessionCoordinator {
     try {
       playback.onFrozen = () => unawaited(reloadVideo(auto: true));
       playback.onYtdlError = (e) => health.value = HealthStatus(HealthSeverity.error, "can't open this link: $e");
+      playback.onLoadError = (e) => health.value = HealthStatus(HealthSeverity.error, "can't play this: $e");
       playback.onUserSeek = (d) => _userSeek(d.inMicroseconds / 1e6);
       await playback.init();
       _playerReady = true;