feat(player): resume on media-button when session torn down (#448)

Fast-follow of #54. After the #52 idle/dismiss teardown, pressing play
on a headset / watch / lock screen did nothing (handler.play() was just
_player.play() with nothing loaded; the handler is Riverpod-agnostic).

- audio_handler: _resumeHook + setResumeHook(); play() is now async —
  when mediaItem == null and a hook is set it awaits the hook (which
  restores + plays) and returns, else _player.play()
- resume_controller: extract shared _loadAndRestore() (bool); _restore()
  keeps the paused launch path; new resumeFromMediaButton() restores
  then starts playback; start() registers it via setResumeHook

Recursion-safe (post-restore mediaItem != null so the re-play hits
_player.play()); no-op when nothing to resume / auth missing / a
session is already active.

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

flutter_client/lib/cache/resume_controller.dart

@@ -45,6 +45,9 @@ class ResumeController with WidgetsBindingObserver {
     await _restore();
     if (_disposed) return;
     final h = _ref.read(audioHandlerProvider);
+    // Media-button-when-torn-down hook (#448): play() invokes this when
+    // mediaItem is null so a headset/watch press resumes the snapshot.
+    h.setResumeHook(resumeFromMediaButton);
     _subs.add(h.mediaItem.listen((_) => _schedulePersist()));
     _subs.add(h.playbackState.listen((_) => _schedulePersist()));
     WidgetsBinding.instance.addObserver(this);
@@ -88,42 +91,63 @@ class ResumeController with WidgetsBindingObserver {
     }
   }
 
+  /// Launch path: restore the last session PAUSED (no auto-blast).
   Future<void> _restore() async {
     try {
-      final h = _ref.read(audioHandlerProvider);
-      // A session already started before restore ran (deep link, etc.)
-      // — don't stomp it.
-      if (h.mediaItem.value != null) return;
-      final url = await _ref.read(serverUrlProvider.future);
-      final token =
-          await _ref.read(secureStorageProvider).read(key: 'session_token');
-      // Streaming needs auth + base URL; nothing to resume without them.
-      if (url == null || url.isEmpty || token == null || token.isEmpty) {
-        return;
-      }
-      final db = _ref.read(appDbProvider);
-      final row = await db.select(db.cachedResumeState).getSingleOrNull();
-      if (row == null) return;
-      final m = jsonDecode(row.json) as Map<String, dynamic>;
-      final rawTracks = (m['tracks'] as List?) ?? const [];
-      final tracks = rawTracks
-          .map((e) => TrackRef.fromJson(e as Map<String, dynamic>))
-          .toList();
-      if (tracks.isEmpty) return;
-      final idx = (m['index'] as num?)?.toInt() ?? 0;
-      final posMs = (m['position_ms'] as num?)?.toInt() ?? 0;
-      final source = m['source'] as String?;
-      await _ref.read(playerActionsProvider).restoreQueue(
-            tracks,
-            initialIndex: idx,
-            position: Duration(milliseconds: posMs),
-            source: source,
-          );
+      await _loadAndRestore();
     } catch (e, st) {
       debugPrint('resume_controller: restore failed: $e\n$st');
     }
   }
 
+  /// Media-button path (#448): the user pressed play on the headset /
+  /// watch / lock screen while the session was fully torn down (#52) and
+  /// nothing is loaded. Restore the snapshot, then START playback (they
+  /// asked to play). Registered as the handler's resume hook in start().
+  /// No-op if there's nothing to resume.
+  Future<void> resumeFromMediaButton() async {
+    try {
+      if (await _loadAndRestore()) {
+        await _ref.read(audioHandlerProvider).play();
+      }
+    } catch (e, st) {
+      debugPrint('resume_controller: media-button resume failed: $e\n$st');
+    }
+  }
+
+  /// Shared loader. Restores the last persisted session PAUSED and
+  /// returns whether it actually restored a queue. Guards: an already-
+  /// active session (don't stomp), missing auth, no/empty snapshot.
+  Future<bool> _loadAndRestore() async {
+    final h = _ref.read(audioHandlerProvider);
+    if (h.mediaItem.value != null) return false;
+    final url = await _ref.read(serverUrlProvider.future);
+    final token =
+        await _ref.read(secureStorageProvider).read(key: 'session_token');
+    if (url == null || url.isEmpty || token == null || token.isEmpty) {
+      return false;
+    }
+    final db = _ref.read(appDbProvider);
+    final row = await db.select(db.cachedResumeState).getSingleOrNull();
+    if (row == null) return false;
+    final m = jsonDecode(row.json) as Map<String, dynamic>;
+    final rawTracks = (m['tracks'] as List?) ?? const [];
+    final tracks = rawTracks
+        .map((e) => TrackRef.fromJson(e as Map<String, dynamic>))
+        .toList();
+    if (tracks.isEmpty) return false;
+    final idx = (m['index'] as num?)?.toInt() ?? 0;
+    final posMs = (m['position_ms'] as num?)?.toInt() ?? 0;
+    final source = m['source'] as String?;
+    await _ref.read(playerActionsProvider).restoreQueue(
+          tracks,
+          initialIndex: idx,
+          position: Duration(milliseconds: posMs),
+          source: source,
+        );
+    return true;
+  }
+
   @override
   void didChangeAppLifecycleState(AppLifecycleState state) {
     // App backgrounded / killed: persist durably right now (the debounce