fix: Ctrl+R flip also matches the physical R key (v1.6.1)

On some keymaps the R key resolves to a different logical key (observed on a
Turkish remote-desktop setup: the R key arrives as logical T), so the Ctrl+R
shortcut never fired even though the key events reached the app. The shortcut
now matches PhysicalKeyboardKey.keyR as well; a regression test covers the
wrong-layout case.

lib/screens/viewer_screen.dart

@@ -190,6 +190,8 @@ class _ViewerScreenState extends State<ViewerScreen> with WindowListener {
     );
   }
 
+  void _flipHorizontal() => _canvasKey.currentState?.toggleFlipH();
+
   Future<void> _cropSave() async {
     final path = ImageManager.instance.currentImagePath;
     final state = _canvasKey.currentState;
@@ -237,6 +239,7 @@ class _ViewerScreenState extends State<ViewerScreen> with WindowListener {
   Widget build(BuildContext context) {
     return AppShortcuts(
       onCropSave: _hasImages ? _cropSave : null,
+      onFlipH: _hasImages ? _flipHorizontal : null,
       child: Scaffold(
         backgroundColor: Colors.transparent,
         body: WindowResizeZones(