feat: window size presets — 4x8 screen-ratio grid in settings (v1.8.0)

- Settings 'Size' section: 8 col x 4 row grid, 31 buttons labeled a:b
  (a splits H, b splits W); top-left cell empty placeholder (no 1:1 —
  title bar double click already maximizes)
- Click resizes the window to work.width/b x work.height/a of the
  display's work area (screen_retriever visibleSize, taskbar excluded);
  position unchanged, no selection state tracking
- minimumSize 400x300 -> 160x100 (4:8 cell needs it)
- Settings file-properties rows left-aligned (were centered under path)

lib/l10n/app_localizations_en.dart

@@ -20,6 +20,9 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get sortByName => 'By name';
 
+  @override
+  String get sizeSection => 'Size';
+
   @override
   String get appDescription =>
       'Ultra-lightweight and fast image viewer. Open images and use arrows or keyboard keys to navigate.';