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/MainFlutterWindow.swift

@@ -1,15 +0,0 @@
-import Cocoa
-import FlutterMacOS
-
-class MainFlutterWindow: NSWindow {
-  override func awakeFromNib() {
-    let flutterViewController = FlutterViewController()
-    let windowFrame = self.frame
-    self.contentViewController = flutterViewController
-    self.setFrame(windowFrame, display: true)
-
-    RegisterGeneratedPlugins(registry: flutterViewController)
-
-    super.awakeFromNib()
-  }
-}