feat(player): periodic PlaybackState refresh for smooth external scrubber

_broadcastState only set updatePosition on transitions, so the lock-
screen / Wear / Android Auto scrubber jumped in chunks (the in-app bar
uses positionStream and was fine). Add _positionBroadcastTimer: a 1s
periodic PlaybackState re-broadcast while actively playing so
updateTime/updatePosition stay fresh and external surfaces interpolate
smoothly. Idempotent (driven from _broadcastState, which the tick
itself calls — guarded against pile-up), cancelled when not playing
and in stop(). 6a: the notification progress bar now advances since
MediaItem.duration was already set.

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

flutter_client/lib/player/audio_handler.dart

@@ -70,6 +70,12 @@ class MinstrelAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandl
   /// cancelled the moment playback resumes or a new queue is set.
   Timer? _idleStopTimer;
 
+  /// Periodic PlaybackState re-broadcast while actively playing so
+  /// external surfaces (lock screen, Wear, Android Auto) interpolate the
+  /// scrubber smoothly. The in-app bar uses positionStream and doesn't
+  /// need this. Active only while playing; cancelled otherwise.
+  Timer? _positionBroadcastTimer;
+
   /// Player volume captured when an OS duck interruption begins,
   /// restored when it ends. Null when not currently ducked.
   double? _volumeBeforeDuck;
@@ -544,6 +550,8 @@ class MinstrelAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandl
   Future<void> stop() async {
     _idleStopTimer?.cancel();
     _idleStopTimer = null;
+    _positionBroadcastTimer?.cancel();
+    _positionBroadcastTimer = null;
     try {
       await _player.stop();
     } catch (_) {}
@@ -712,6 +720,7 @@ class MinstrelAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandl
       },
     ));
     _reconcileIdleTimer();
+    _reconcilePositionBroadcast();
   }
 
   /// Arms (or cancels) the idle-cleanup timer based on the current
@@ -746,6 +755,28 @@ class MinstrelAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandl
     unawaited(stop());
   }
 
+  /// While actively playing, keeps a 1s periodic PlaybackState re-
+  /// broadcast running so updateTime/updatePosition stay fresh and
+  /// external surfaces interpolate the scrubber smoothly. Idempotent and
+  /// driven from _broadcastState — which the periodic tick itself calls,
+  /// so the "already running" guard prevents pile-up. Cancels the moment
+  /// playback is no longer active.
+  void _reconcilePositionBroadcast() {
+    final ps = _player.processingState;
+    final active = _player.playing &&
+        ps != ProcessingState.idle &&
+        ps != ProcessingState.completed;
+    if (!active) {
+      _positionBroadcastTimer?.cancel();
+      _positionBroadcastTimer = null;
+      return;
+    }
+    _positionBroadcastTimer ??= Timer.periodic(
+      const Duration(seconds: 1),
+      (_) => _broadcastState(null),
+    );
+  }
+
   /// Configures the OS audio session for music playback and wires
   /// interruption + becoming-noisy handling. just_audio auto-activates /
   /// deactivates the session around playback once it's configured, but