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/controls/uosc/controls.dart

@@ -6,11 +6,10 @@ import 'package:signals/signals_flutter.dart';
 import '../../state/settings.dart';
 
 // The experimental mpv-style (uosc) controls frame: a full-width timeline
-// at the bottom with a row of flat buttons above it, window buttons top
+// at the bottom with a row of flat buttons above it, a corner widget top
 // right. Mouse movement shows them; after [UoscControls.hideAfter] without
 // any they fade out, the timeline shrinks to a thin line and the cursor
-// hides. Everything lives here because media_kit's fullscreen route
-// renders only the video and its controls.
+// hides. Our own frame: no media_kit controls state to reach into.
 
 class UoscControls extends StatefulWidget {
   /// The timeline; [expanded] is false while the controls are hidden.
@@ -18,7 +17,7 @@ class UoscControls extends StatefulWidget {
   /// Buttons left and right of the row above the timeline.
   final List<Widget> left;
   final List<Widget> right;
-  /// Top-right corner (window buttons), shown with the controls.
+  /// Top-right corner (chat, back to the library), shown with the controls.
   final Widget? top;
   final VoidCallback? onDoubleTap;
   final Duration hideAfter;