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/screens/home/settings_page.dart

@@ -7,11 +7,12 @@ import '../../update/update_controller.dart';
 import 'ui.dart';
 import 'update_indicator.dart';
 
-/// Player keys, as handled by the session player's key handler (while
-/// the player fills the window).
+/// Session keys: Space and Tab anywhere in a server (`AppKeys`), the
+/// rest by the session player's key handler (while it fills the window).
 const sessionShortcuts = [
   ('Space', 'Toggle ready'),
   ('← / →', 'Seek back / forward 10 s'),
+  ('↑ / ↓', 'Volume up / down (up to 150 %)'),
   ('Shift+→', 'Skip 1:30 (for everyone)'),
   ('PgUp / PgDn', 'Previous / next chapter'),
   ('F', 'Toggle fullscreen'),