Protocol 10 presence gate, drop/paste anywhere, quick UI fixes

Ready gate: whoever is in the server while it is not playing counts (joining
while playing does not pause anyone); a swap counts everyone present and
keeps ready flags; the quorum rule is gone. Optional 'Unready when the
video changes'. No 'Skip 1:30' guess when the media has chapters.

Drag & drop and Ctrl+V anywhere (fox_utils): files, folders and links get a
small card asking how to open them. Files series start collapsed.

UI: Tab never moves focus and Space is always ready outside text fields;
chat focuses its field; unread badge on uosc and a chip in fullscreen;
people-only chat with an Activity toggle; skip pills show 5 s then with
the controls; volume up to 150%; own avatar same size as the party.

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

apps/desktop/lib/src/playback/playback_controller.dart

@@ -32,6 +32,9 @@ class PlaybackController {
   /// Back buffer (media_kit sets it to bufferSize too): enough for a short
   /// seek back without holding another 256 MiB.
   static const maxBackBytes = 64 * 1024 * 1024;
+  /// mpv's `volume-max` (its default is 130): the volume controls go
+  /// this far, past 100 % amplified.
+  static const maxVolume = 150;
 
   SyncPlayer? _player;
   VideoController? _video;
@@ -163,6 +166,7 @@ class PlaybackController {
     _logSub = _player!.stream.log.listen(_onLog);
     _watchMarks(_player!);
     await _pinBufferProps();
+    await _pin('volume-max', '$maxVolume');
     await applyShaders();
     _watchdog.reset();
     _watchTimer = Timer.periodic(const Duration(seconds: 1), (_) => _watch());