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/uosc_controls_test.dart

@@ -11,9 +11,9 @@ import 'package:watch_together/src/controls/uosc/timeline.dart';
 import 'package:watch_together/src/jellyfin/track_prefs.dart';
 import 'package:watch_together/src/playback/media_marks.dart';
 import 'package:watch_together/src/playback/playback_controller.dart' show RenderMode;
-import 'package:watch_together/src/screens/home/settings_page.dart';
 import 'package:watch_together/src/screens/session/player_controls.dart' show TrackPick;
 import 'package:watch_together/src/shaders/shader_presets.dart';
+import 'package:watch_together/src/shell/settings_tab.dart';
 import 'package:watch_together/src/state/settings.dart';
 import 'package:watch_together/src/update/update_controller.dart';
 
@@ -456,7 +456,13 @@ void main() {
     addTearDown(tester.view.reset);
     await tester.pumpWidget(MaterialApp(
       theme: ThemeData.dark(),
-      home: Scaffold(body: SettingsPage(updates: UpdateController(currentVersion: '0.0.0-dev'))),
+      home: Scaffold(
+        body: SettingsTab(
+          section: SettingsSection.player,
+          onSection: (_) {},
+          updates: UpdateController(currentVersion: '0.0.0-dev'),
+        ),
+      ),
     ));
     final toggle = find.byKey(const ValueKey('mpv-controls'));
     expect(find.text('Experimental: mpv-style controls'), findsOneWidget);