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/shell/library_link.dart

@@ -346,7 +346,7 @@ class _LibraryLinkSourceState extends State<LibraryLinkSource> {
               Text('Watch from a link', textAlign: TextAlign.center, style: t.headlineSmall),
               const SizedBox(height: 6),
               Text(
-                'A video file, YouTube, a Jellyfin link… Ctrl+V anywhere pastes it here.',
+                'A video file, YouTube, a Jellyfin link… Ctrl+V or drop it anywhere to bring it here.',
                 textAlign: TextAlign.center,
                 style: TextStyle(color: scheme.onSurfaceVariant),
               ),