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/main.dart

@@ -18,7 +18,7 @@ void main(List<String> args) async {
 
   final windowOptions = WindowOptions(
     size: saved?.size ?? defaultSize,
-    minimumSize: const Size(400, 300),
+    minimumSize: const Size(160, 100),
     backgroundColor: Colors.transparent,
     titleBarStyle: TitleBarStyle.hidden,
   );