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/lib/app.dart

@@ -1,28 +1,17 @@
 import 'package:flutter/material.dart';
-import 'package:signals/signals_flutter.dart';
 
-import 'src/screens/home/home_screen.dart';
 import 'src/screens/home/ui.dart';
-import 'src/screens/session_screen.dart';
-import 'src/state/session_state.dart';
+import 'src/shell/app_shell.dart';
 
 class WatchTogetherApp extends StatelessWidget {
   const WatchTogetherApp({super.key});
 
   @override
   Widget build(BuildContext context) {
-    final session = SessionState.instance;
     return MaterialApp(
       title: appName,
       theme: ThemeData.dark(),
-      home: SignalBuilder(
-        builder: (context) {
-          return switch (session.route.value) {
-            AppRoute.home => const HomeScreen(),
-            AppRoute.session => const SessionScreen(),
-          };
-        },
-      ),
+      home: const AppShell(),
     );
   }
 }