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/windows/runner/flutter_window.h

@@ -1,33 +0,0 @@
-#ifndef RUNNER_FLUTTER_WINDOW_H_
-#define RUNNER_FLUTTER_WINDOW_H_
-
-#include <flutter/dart_project.h>
-#include <flutter/flutter_view_controller.h>
-
-#include <memory>
-
-#include "win32_window.h"
-
-// A window that does nothing but host a Flutter view.
-class FlutterWindow : public Win32Window {
- public:
-  // Creates a new FlutterWindow hosting a Flutter view running |project|.
-  explicit FlutterWindow(const flutter::DartProject& project);
-  virtual ~FlutterWindow();
-
- protected:
-  // Win32Window:
-  bool OnCreate() override;
-  void OnDestroy() override;
-  LRESULT MessageHandler(HWND window, UINT const message, WPARAM const wparam,
-                         LPARAM const lparam) noexcept override;
-
- private:
-  // The project to run.
-  flutter::DartProject project_;
-
-  // The Flutter instance hosted by this window.
-  std::unique_ptr<flutter::FlutterViewController> flutter_controller_;
-};
-
-#endif  // RUNNER_FLUTTER_WINDOW_H_