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/shell/motion.dart

@@ -0,0 +1,126 @@
+import 'package:flutter/gestures.dart';
+import 'package:flutter/material.dart';
+
+// The shell's shared motion: one curve and one duration for everything
+// that moves, a fade-through for views that replace each other, popovers
+// that grow from where they open, and mouse-friendly horizontal lists.
+
+const ease = Curves.fastEaseInToSlowEaseOut;
+const easeTime = Duration(milliseconds: 320);
+
+/// Fade-through length; the old view is gone after [fadeOutPart] of it.
+const fadeThroughTime = Duration(milliseconds: 360);
+const fadeOutPart = 0.35;
+
+/// Fade through (Material motion): the old view is gone after the first
+/// 35%, then the new one fades and scales in; they never overlap. Key
+/// [child] by what it shows so a change of key runs the transition.
+Widget fadeThrough(Widget child, {Key? key}) => AnimatedSwitcher(
+      key: key,
+      duration: fadeThroughTime,
+      layoutBuilder: (current, previous) => Stack(
+        fit: StackFit.expand,
+        children: [...previous, ?current],
+      ),
+      transitionBuilder: (c, anim) {
+        if (c.key == child.key) {
+          // Incoming (anim 0 -> 1): wait, then fade and scale up.
+          final a = CurvedAnimation(
+            parent: anim,
+            curve: const Interval(fadeOutPart, 1, curve: ease),
+          );
+          return FadeTransition(
+            opacity: a,
+            child: ScaleTransition(scale: Tween(begin: 0.985, end: 1.0).animate(a), child: c),
+          );
+        }
+        // Outgoing (anim 1 -> 0): fully faded by the time it reaches 0.65.
+        return FadeTransition(
+          opacity: CurvedAnimation(
+            parent: anim,
+            curve: const Interval(1 - fadeOutPart, 1, curve: Curves.easeIn),
+          ),
+          child: c,
+        );
+      },
+      child: child,
+    );
+
+/// A card over the window that fades and scales in from [at] (window
+/// alignment) instead of popping; a click outside or Esc closes it. Short
+/// windows: the card scrolls instead of overflowing.
+Future<T?> showPopover<T>(
+  BuildContext context, {
+  required Alignment at,
+  required Widget child,
+  bool dim = false,
+}) =>
+    showGeneralDialog<T>(
+      context: context,
+      barrierDismissible: true,
+      barrierLabel: 'close',
+      barrierColor: dim ? Colors.black26 : Colors.transparent,
+      transitionDuration: easeTime,
+      pageBuilder: (context, _, _) => SafeArea(
+        child: Align(
+          alignment: at,
+          child: ConstrainedBox(
+            constraints: BoxConstraints(maxHeight: MediaQuery.sizeOf(context).height - 32),
+            child: SingleChildScrollView(child: child),
+          ),
+        ),
+      ),
+      transitionBuilder: (context, anim, _, child) {
+        final a = CurvedAnimation(parent: anim, curve: ease, reverseCurve: Curves.easeIn);
+        return FadeTransition(
+          opacity: a,
+          child: ScaleTransition(
+            scale: Tween(begin: 0.96, end: 1.0).animate(a),
+            alignment: at,
+            child: child,
+          ),
+        );
+      },
+    );
+
+/// Horizontal lists for mouse users: drag with the mouse, and the plain
+/// (vertical) wheel moves them sideways. At either end the wheel is left
+/// to the page, so a row never traps vertical scrolling.
+class HScroll extends StatefulWidget {
+  final ScrollController? controller;
+  final Widget Function(ScrollController controller) builder;
+  const HScroll({super.key, this.controller, required this.builder});
+
+  @override
+  State<HScroll> createState() => _HScrollState();
+}
+
+class _HScrollState extends State<HScroll> {
+  ScrollController? _own;
+  ScrollController get _c => widget.controller ?? (_own ??= ScrollController());
+
+  @override
+  void dispose() {
+    _own?.dispose();
+    super.dispose();
+  }
+
+  void _wheel(PointerSignalEvent e) {
+    if (e is! PointerScrollEvent) return;
+    final dy = e.scrollDelta.dy;
+    if (dy == 0 || e.scrollDelta.dx != 0 || !_c.hasClients) return;
+    final p = _c.position;
+    final target = (p.pixels + dy * 1.5).clamp(p.minScrollExtent, p.maxScrollExtent);
+    if (target == p.pixels) return; // at the end: let the page scroll
+    GestureBinding.instance.pointerSignalResolver.register(e, (_) {
+      _c.animateTo(target, duration: const Duration(milliseconds: 140), curve: Curves.easeOutCubic);
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) => ScrollConfiguration(
+        behavior: ScrollConfiguration.of(context)
+            .copyWith(dragDevices: PointerDeviceKind.values.toSet()),
+        child: Listener(onPointerSignal: _wheel, child: widget.builder(_c)),
+      );
+}