Fix mobile app submodule initialization and cleanup

- Removed incorrect swingmusic_mobile directory
- Properly initialized swingmusic_mobile submodule from swingmusic-mobile.git
- Clean submodule configuration for unified release workflow

swingmusic_mobile/macos/Runner/AppDelegate.swift

@@ -1,13 +0,0 @@
-import Cocoa
-import FlutterMacOS
-
-@main
-class AppDelegate: FlutterAppDelegate {
-  override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
-    return true
-  }
-
-  override func applicationSupportsSecureRestorableState(_ app: NSApplication) -> Bool {
-    return true
-  }
-}