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_en.arb

@@ -4,6 +4,7 @@
   "sortBy": "Sort by",
   "sortByTime": "By time",
   "sortByName": "By name",
+  "sizeSection": "Size",
   "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+O — open file\n• Ctrl+S — save visible area (JPEG 95)\n• Ctrl+R — flip horizontally (clears effects, press again to undo)\n• Ctrl+Q — close window\n• Ctrl+Shift+Q — close all windows\n• F11 — toggle fullscreen\n• Esc — exit fullscreen\n• Scroll — zoom (10% per step)\n• Ctrl+scroll — fine zoom (1% per step)\n• Left-drag — pan\n• Double click — toggle fill/fit\n• Right double click — reset effects\n• Right-drag — contrast\n• Shift+right-drag — rotate\n• Ctrl+right-drag — saturation\n• Ctrl+Shift+right-drag — brightness",