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/l10n/app_en.arb

@@ -6,7 +6,7 @@
   "sortByName": "By name",
   "appDescription": "Ultra-lightweight and fast image viewer. Open images and use arrows or keyboard keys to navigate.",
   "shortcuts": "Shortcuts",
-  "shortcutsList": "• Left/Right arrow — previous/next image\n• Ctrl+S — save visible area (JPEG 93)\n• Ctrl+Q — close window\n• Ctrl+Shift+Q — close all windows\n• Double click — toggle fill/fit\n• Right-drag — contrast\n• Shift+right-drag — rotate\n• Ctrl+right-drag — saturation\n• Ctrl+Shift+right-drag — brightness\n• R — flip horizontally (resets effects, press again to undo)",
+  "shortcutsList": "• Left/Right arrow — previous/next image\n• Ctrl+S — save visible area (JPEG 93)\n• Ctrl+R — flip horizontally (clears effects, press again to undo)\n• Ctrl+Q — close window\n• Ctrl+Shift+Q — close all windows\n• Double click — toggle fill/fit\n• Right-drag — contrast\n• Shift+right-drag — rotate\n• Ctrl+right-drag — saturation\n• Ctrl+Shift+right-drag — brightness",
   "print": "Print",
   "close": "Close",
   "saveCrop": "Save visible area (JPEG 93)",