MacOS: Hỗ trợ phím Fn (Globe) và sửa một vài lỗi vặt

[luatnd]

New feat

• Hỗ trợ chuyển E <-> V chỉ với 1 phím 🌐 Fn (Globe) như mặc định của MacOS mới, không cần nhấm tổ hợp phím nữa. Fn (🌐) single-press as language switch on modern Mac keyboards
  • New HAS_FN bit (0x1000) in the hotkey bitfield (Engine.h)
  • Fn-flag check in checkHotKey() (OpenKey.mm:503-504)
  • "🌐" checkbox in preferences (ViewController.m + storyboard)
  • Italic hint label about the System Settings → Keyboard → "Press 🌐: Do Nothing" requirement

Fix

1. Lỗi không hiển thị UI do nhớ nhầm vị trí Bảng điều khiễn cũ ==> Căn vị trí mới khi hiển thị Bảng điều khiển. UI hang / unresponsive preferences — initEventTap was calling CFRunLoopRun() on the main thread, starting a nested loop that blocked AppKit from processing events (OpenKeyManager.m). Latent bug independent of the Fn feature.
2. Preferences window invisible on macOS Tahoe — LSUIElement=true apps now need [NSApp activateIgnoringOtherApps:YES] before makeKeyAndOrderFront:, otherwise windows go behind the foreground app (AppDelegate.m).
3. Window restored off-screen — saved frame from a prior multi-display setup placed the window at (1932, -804) (a disconnected external monitor). Added re-centering on [NSScreen screens].firstObject (AppDelegate.m).
4. Storyboard layout overlap — first attempt placed the Fn checkbox on top of the existing "Chế độ gõ" radio row. Expanded the "Điều khiển" box from 112→137px, shifted the upper rows up, added a dedicated row for the hint label.