Add support for fusing/unfusing JellyLMS zones into synchronized

multi-room groups, addressed by MAC (derived from the `lms-{mac}` device id).

src-tauri/src/playback_mode/mod.rs

@@ -81,6 +81,19 @@ impl PlaybackModeManager {
         self.is_transferring.load(Ordering::Relaxed)
     }
 
+    /// Set the transferring flag directly.
+    ///
+    /// The remote->local transfer is driven from the frontend in two steps
+    /// (`player_play_tracks` to start local playback, then
+    /// `playback_mode_transfer_to_local` to stop the remote). The first step's
+    /// routing depends on this flag: while it's set, `player_play_tracks` plays
+    /// locally instead of casting back to the remote session. The frontend must
+    /// raise the flag *before* that first call and lower it when the sequence is
+    /// done (or aborts), so it can't be left stuck on.
+    pub fn set_transferring(&self, transferring: bool) {
+        self.is_transferring.store(transferring, Ordering::Relaxed);
+    }
+
     /// Send volume command to remote session
     /// Commands: "SetVolume", "VolumeUp", "VolumeDown"
     #[allow(dead_code)] // Called from Android JNI callback