Restructure step 1: lasting identity, presence and protocol 16

Server and core: version ranges on both sides with a coded refusal; a
client id maps to the same member and token for the server's life; a
drop is `gone` for a 10-minute grace keeping ready, presence, queue
items, relay and avatar; `bye` leaves at once; ordered takeover (seq,
replaced/stale); presence joining/in/away/gone with one PresencePolicy;
joiners never pause playback and hold a held gate at most ~15 s;
state.hold; feed ids, seq and replay; the relay lives through its
source's grace and continues on re-dial; queue `by` lasting, auto
items and cancel; MediaRef carried; client ids never logged.

App: persisted client id and seq; Connection with unstable/lost/
replaced states, backoff with jitter, no lost/back pair per flap; a drop
keeps the session (state, player, media, relay source); presence in the
strip and members card; sit out = away intent; feed matched by id;
auto sit-out (idle when not playing, minimised for a while; never when
alone) replaces auto-unready; bye on close and Leave, not on update
restart; relay source re-dials through its grace.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/lib/src/shell/app_shell.dart

@@ -16,7 +16,7 @@ import '../session/app_events.dart' show ActionFailed, ScheduleResult;
 import '../session/session_coordinator.dart' show ConnectParams;
 import '../screens/home/health_probe.dart';
 import '../screens/home/media_picker.dart';
-import '../screens/home/profile.dart';
+import '../session/identity.dart';
 import '../screens/home/update_indicator.dart';
 import '../state/session_state.dart';
 import '../state/settings.dart';
@@ -202,7 +202,7 @@ class AppShellState extends State<AppShell> with SingleTickerProviderStateMixin
     // plays again).
     var wasOut = false;
     _stops.add(effect(() {
-      final out = coord.selfOut.value;
+      final out = coord.selfAway.value;
       if (wasOut && !out && session.route.peek() == AppRoute.session) {
         _later(() {
           if (_mode == PlayerMode.none) _show(PlayerMode.mini);
@@ -323,7 +323,7 @@ class AppShellState extends State<AppShell> with SingleTickerProviderStateMixin
   /// player shows the join-back card). Once out, × closes the player.
   void _closePlayer() {
     final coord = SessionState.instance.coordinator;
-    if (coord.selfOut.peek()) {
+    if (coord.selfAway.peek()) {
       _show(PlayerMode.none);
     } else {
       coord.sitOut();
@@ -447,6 +447,8 @@ class AppShellState extends State<AppShell> with SingleTickerProviderStateMixin
 
   /// Ctrl+V outside a text field (a focused field pastes on its own).
   bool _onKey(KeyEvent e) {
+    // Any key is activity (auto sit-out).
+    SessionState.instance.coordinator.markInput();
     if (e is! KeyDownEvent || e.logicalKey != LogicalKeyboardKey.keyV) return false;
     final keys = HardwareKeyboard.instance;
     if (!(keys.isControlPressed || keys.isMetaPressed) || keys.isShiftPressed || keys.isAltPressed) return false;
@@ -675,9 +677,18 @@ class AppShellState extends State<AppShell> with SingleTickerProviderStateMixin
         }),
       ),
     );
+    // Any pointer use anywhere is activity (auto sit-out).
+    final coord = SessionState.instance.coordinator;
     return Listener(
-      onPointerHover: (e) => _pointer = e.localPosition,
-      onPointerDown: (e) => _pointer = e.localPosition,
+      onPointerHover: (e) {
+        _pointer = e.localPosition;
+        coord.markInput();
+      },
+      onPointerDown: (e) {
+        _pointer = e.localPosition;
+        coord.markInput();
+      },
+      onPointerSignal: (_) => coord.markInput(),
       child: _dropTarget(shell),
     );
   }