feat(flutter): #426B client — offline play capture via mutation queue

Flutter half of offline-replay capture. Play events no longer
fire-and-forget: the reporter now tracks each play as a completed
unit (track, original start time, source, duration reached)
independently of connectivity.

- EventsApi.playOffline: single timestamp-preserving call → the new
  /api/events play_offline (47aa178).
- MutationQueue: new play.offline kind + handler (EventsApi).
- PlayEventsReporter rework:
  - _beginTrack captures start context + fires live play_started;
    the server id is adopted only if it lands while still on-track.
  - position progress gated on the tracked track id so a track
    change can't clobber the finishing track's last values.
  - _closeCurrent: if a server id registered, attempt the live
    ended/skipped and fall back to the offline queue on failure; if
    no id (offline start) enqueue the completed play directly. The
    server applies the canonical skip rule, so the offline payload
    only carries duration.
  - app paused/detached closes durably via the queue (survives a
    process kill; a teardown POST would not).

Result: listening to cached tracks fully offline now records
history / recs / scrobble / #415 rotation once back online, with
the original timestamps. Web stays best-effort by standing
occasional-use scope.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/cache/mutation_queue.dart

@@ -30,6 +30,7 @@ import 'package:flutter/foundation.dart' show debugPrint;
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../api/endpoints/discover.dart';
+import '../api/endpoints/events.dart';
 import '../api/endpoints/likes.dart';
 import '../library/library_providers.dart' show dioProvider;
 import '../likes/likes_provider.dart' show likesApiProvider;
@@ -52,6 +53,7 @@ class MutationKinds {
   static const playlistAppend = 'playlist.append';
   static const requestCreate = 'request.create';
   static const requestCancel = 'request.cancel';
+  static const playOffline = 'play.offline';
 }
 
 class MutationQueue {
@@ -237,6 +239,8 @@ typedef _Handler = Future<void> Function(Ref, Map<String, dynamic>);
 /// * playlist.append: {'playlistId': uuid, 'trackIds': [uuid, …]}
 /// * request.create: {full createRequest args; see DiscoverApi}
 /// * request.cancel: {'id': uuid}
+/// * play.offline: {'trackId': uuid, 'clientId': str, 'at': iso8601,
+///   'durationPlayedMs': int, 'source'?: 'for_you'|'discover'}
 final Map<String, _Handler> _handlers = {
   MutationKinds.likeAdd: (ref, p) async {
     final api = await ref.read(likesApiProvider.future);
@@ -281,6 +285,17 @@ final Map<String, _Handler> _handlers = {
     final api = await ref.read(requestsApiProvider.future);
     await api.cancel(p['id'] as String);
   },
+  MutationKinds.playOffline: (ref, p) async {
+    final dio = await ref.read(dioProvider.future);
+    final api = EventsApi(dio);
+    await api.playOffline(
+      trackId: p['trackId'] as String,
+      clientId: p['clientId'] as String,
+      atIso: p['at'] as String,
+      durationPlayedMs: (p['durationPlayedMs'] as num).toInt(),
+      source: p['source'] as String?,
+    );
+  },
 };
 
 LikeKind _likeKindFromString(String s) => switch (s) {