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>

packages/server/test/session_test.dart

@@ -80,8 +80,8 @@ void main() {
   });
 
   group('core welcome + proto', () {
-    test('proto 9: avatars by hash, profile intent', () {
-      expect(RelayConstants.protoVersion, 9);
+    test('proto 10: avatars by hash, profile intent', () {
+      expect(RelayConstants.protoVersion, 10);
       const v = MemberView(id: 'm1', name: 'a', ready: false, inSet: false, buffered: 0, stale: false);
       expect(v.toJson().containsKey('avatarHash'), isFalse);
       expect(v.toJson().containsKey('avatar'), isFalse);