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/src/sync/sync_client.dart

@@ -33,6 +33,8 @@ class SyncClient {
   Stream<String> get errors => _errors.stream;
   Stream<SyncStatus> get status => _status.stream;
 
+  /// The `/sync` URL last dialled; its host serves `/a/...` and the media.
+  Uri? wsUri;
   String token = '';
   String memberId = '';
   String? mediaUrl;
@@ -53,6 +55,7 @@ class SyncClient {
   }) async {
     await disconnect();
     clockSync.reset();
+    this.wsUri = wsUri;
     _status.add(SyncStatus.connecting);
     final ch = WebSocketChannel.connect(wsUri);
     unawaited(ch.ready.then((_) {}, onError: (_) {}));
@@ -240,6 +243,13 @@ class SyncClient {
     _send(ChatMessage(text: clean).toJson());
   }
 
+  /// Mid-session name/picture change; null leaves it, an empty [avatar]
+  /// (base64 PNG) removes the picture.
+  void sendProfile({String? name, String? avatar}) {
+    if (name == null && avatar == null) return;
+    _send(ProfileIntent(name: name, avatar: avatar).toJson());
+  }
+
   void sendKick(String target, {String reason = ''}) {
     _send(KickRequest(target: target, reason: reason).toJson());
   }