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/test/session_logic_test.dart

@@ -91,4 +91,14 @@ void main() {
       expect(n.contains('SECRET'), isFalse);
     });
   });
+
+  test('unreadyForNewMedia: only a real swap while ready, setting on', () {
+    bool u({bool enabled = true, int last = 2, int id = 3, bool ready = true}) =>
+        unreadyForNewMedia(enabled: enabled, lastMediaId: last, mediaId: id, ready: ready);
+    expect(u(), isTrue);
+    expect(u(enabled: false), isFalse);
+    expect(u(ready: false), isFalse);
+    expect(u(id: 2), isFalse, reason: 'same media');
+    expect(u(last: -1), isFalse, reason: 'first state of a session is no swap');
+  });
 }