1.0 shell: one window with a party strip; protocol 9 profile + avatars by hash

The shell (lib/src/shell/) replaces the Home and session screens:
- Party strip: profile, Library / Schedule / Settings tabs, Join and
  Host cards (hosting no longer takes media), status-ring avatar stack
  with the member list, chat & activity panel, Leave.
- The real session player is mounted once and shown full, as a
  draggable corner-snapping mini player, or fullscreen (window_manager,
  party and toasts overlaid); the Video is never mounted twice.
- Profile popover with inline rename, colour and a crop picker (256 px
  PNG), pushed live to the session; avatar toasts; waiting pill.
- Motion kit (fastEaseInToSlowEaseOut, fade-through, popovers,
  mouse-drag/wheel horizontal scrolling).
- Library = the existing picker for now (restyled next round); Schedule
  = the new SchedulePage; Settings = left nav over the existing panels.

Protocol 9 (packages/core, packages/server, sync client):
- {"t":"profile"} renames / changes the picture mid-session ("rename",
  "avatar" feed actions); names are cleaned the same way at hello.
- state.members carry avatarHash; pictures are served once from
  /a/<sid>/<token>/<hash> (PNG/JPEG/WebP/GIF by magic bytes only) and
  cached by hash in the app (AvatarCache).

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

apps/desktop/test/jellyfin_accounts_test.dart

@@ -7,7 +7,6 @@ import 'package:sync_relay_core/core.dart';
 import 'package:watch_together/src/jellyfin/accounts.dart';
 import 'package:watch_together/src/jellyfin/format.dart';
 import 'package:watch_together/src/jellyfin/sign_in.dart';
-import 'package:watch_together/src/screens/home/host_tab.dart' show mediaSummary;
 import 'package:watch_together/src/screens/home/media_picker.dart';
 
 /// Captured Jellyfin 10.11 answers shared with packages/core's client tests.
@@ -327,9 +326,8 @@ void main() {
       expect(back, m);
       expect(back.runtime, const Duration(minutes: 23));
       expect(back.subtitle, 'Black Summoner · S1E6');
-      expect(mediaSummary(m), 'Playing: Rice and Thieves and Heroes — Black Summoner · S1E6');
-      expect(mediaSummary(const FileMedia('/a/b/movie.mkv')), 'Playing: movie.mkv');
-      expect(mediaSummary(null), 'No media selected (optional).');
+      expect(m.label, 'Rice and Thieves and Heroes — Black Summoner · S1E6');
+      expect(const FileMedia('/a/b/movie.mkv').label, 'movie.mkv');
       expect(JellyfinMedia.fromJson({'serverId': 1}), isNull);
     });