Fix intermittent app freeze when toggling fullscreen (F11 / player button)

[Copilot]

Rapid or concurrent fullscreen toggles (F11, Alt+Enter, player button) could send conflicting commands to the window manager before any single transition completed, causing the app to freeze — particularly on Windows with hybrid GPU setups.

Two bugs in AnymexTitleBar (lib/widgets/custom_widgets/anymex_titlebar.dart):

• Concurrent calls not guarded: multiple in-flight windowManager.setFullScreen() calls could race against each other
• Stale cached state: toggleFullScreen() read from isFullScreen.value (local cache) instead of the actual OS window state, so any drift caused every subsequent toggle to issue the wrong command

Changes

• AnymexTitleBar (anymex_titlebar.dart)
  • Added _isTransitioning boolean guard — any call arriving while a transition is in progress is dropped immediately, preventing duplicate/conflicting window manager calls
  • Both setFullScreen() and toggleFullScreen() now call await windowManager.isFullScreen() to read actual OS state rather than the cached value — once before toggling, once after to confirm final state
  • Guard cleared via try/finally to prevent permanent lock on exception

// Before
static Future<void> toggleFullScreen() async {
  await windowManager.setFullScreen(!isFullScreen.value); // stale cache
  isFullScreen.value = !isFullScreen.value;              // assumes success
}

// After
static Future<void> toggleFullScreen() async {
  if (_isTransitioning) return;                          // drop concurrent calls
  _isTransitioning = true;
  try {
    final currentState = await windowManager.isFullScreen(); // actual OS state
    await windowManager.setFullScreen(!currentState);
    isFullScreen.value = await windowManager.isFullScreen(); // confirm result
  } finally {
    _isTransitioning = false;
  }
}

Type of Changes

• Bug Fix

Testing Notes

Manually verify: rapid F11 presses, clicking the player fullscreen button multiple times in quick succession, and Alt+Enter should no longer freeze the app.

Linked Issue(s)

Additional Context

Most reproducible on Windows with AMD integrated/discrete GPU combos (e.g. AMD Ryzen 5 5500U + Radeon), where the Windows DWM is slower to process fullscreen transitions.

Submission Checklist

• I have read and followed the project's contributing guidelines
• My code follows the code style of this project
• I have tested the changes and ensured they do not break existing functionality
• I have added or updated documentation as needed
• I have linked related issues in the description above
• I have tagged the appropriate reviewers for this pull request