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/queue/queue_controller.dart

@@ -93,7 +93,7 @@ class QueueController {
     effect(() {
       shownContinuation.value;
       coord.settledMediaId.value;
-      coord.selfOut.value;
+      coord.selfAway.value;
       final r = coord.serverState.value;
       untracked(() => _warm(r));
     });
@@ -107,6 +107,7 @@ class QueueController {
   Future<String?> add(PickedMedia pick, {bool next = false}) async {
     coord.markInput();
     if (coord.serverState.peek() == null) return 'not in a server';
+    if (coord.stateStale.peek()) return 'reconnecting — add it again once back';
     if (items.peek().length >= RelayConstants.maxQueueItems) {
       return 'Up next is full (${RelayConstants.maxQueueItems} items)';
     }
@@ -490,7 +491,7 @@ class QueueController {
         if (local == null) return "it isn't on this computer any more";
         // Only this app can pass it on, and a member sitting out has
         // nothing open: back first.
-        if (coord.selfOut.peek()) coord.joinBack();
+        if (coord.selfAway.peek()) coord.joinBack();
         return coord.swapPicked(local.pick, queueId: item.id);
     }
   }
@@ -528,7 +529,7 @@ class QueueController {
   /// puts up (its swap reads it). Null: nothing.
   _WarmTarget? _warmTarget(StateBroadcast? r) {
     final t = _warmTargetHere(r);
-    if (t == null || !coord.selfOut.peek()) return t;
+    if (t == null || !coord.selfAway.peek()) return t;
     return t.probe == null ? null : _WarmTarget(t.id, probe: t.probe);
   }
 
@@ -718,7 +719,7 @@ class QueueController {
     final seen = _lastState;
     if (r == null || (seen != null && seen.mediaId > r.mediaId)) r = seen;
     if (r == null) return;
-    final out = coord.selfOut.peek();
+    final out = coord.selfAway.peek();
     if (!out && r.media != null && coord.settledMediaId.peek() != r.mediaId) return;
     final want = out ? null : _preloadWant;
     final playback = coord.playback;