Fix Android quick actions and Windows taskbar activation

[Treewinds]

Summary

Fixes two platform-specific launch and activation issues:

• Android Quick Settings actions could show a toast without actually starting or stopping the service.
• On Windows, clicking the pinned taskbar icon could not restore an already-running hidden FlClash window.

Root cause

Android

The tile plugin sent the action to Dart without waiting for a result. When the Flutter engine existed but the tile listener was not ready, the action was silently dropped.

Windows

The application did not implement single-instance activation. Clicking the pinned icon launched another process, but that process could not notify the existing hidden window to show itself.

Changes

Android

• Wait for the Dart tile handler to return its result.
• Fall back to the native service path when Dart does not handle the action.
• Keep the quick-action activity alive until the asynchronous action finishes.
• Report whether the Dart-side start or stop operation was handled successfully.

Windows

• Add a named mutex to enforce a single application instance.
• Send a registered activation message to the existing window.
• Forward the activation event through window_ext.
• Show the hidden FlClash window when the activation event is received.
• Use a FlClash-specific Win32 window class name.

Validation

• Dart static analysis passed with no issues.
• Android release Kotlin compilation passed.
• Windows window_ext plugin and runner compiled successfully with MSVC:
  • 0 warnings
  • 0 errors
• git diff --check passed.