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.dart

@@ -122,6 +122,12 @@ abstract class AppLocalizations {
   /// **'By name'**
   String get sortByName;
 
+  /// No description provided for @sizeSection.
+  ///
+  /// In en, this message translates to:
+  /// **'Size'**
+  String get sizeSection;
+
   /// No description provided for @appDescription.
   ///
   /// In en, this message translates to: