fix(01-01): add mutex protection to Queue, Library, and Playlist SetContext methods

- Queue.SetContext acquires existing q.mu before writing q.ctx
- Library struct gets new mu sync.Mutex; SetContext and SetRescanHooks acquire it
- Playlist Service struct gets new mu sync.Mutex; SetContext and SetFavoritesConfig acquire it
- Library and Playlist release lock before calling post-init methods (registerEventHandlers, migrateExistingPlaylists)

backend/queue/queue.go

@@ -132,6 +132,9 @@ func NewQueue(logger *slog.Logger, db *database.DB) *Queue {
 
 // SetContext sets the Wails runtime context for event emission.
 func (q *Queue) SetContext(ctx context.Context) {
+	q.mu.Lock()
+	defer q.mu.Unlock()
+
 	q.ctx = ctx
 }