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/services/window_persistence.dart

@@ -84,6 +84,36 @@ class WindowPersistence {
     }
   }
 
+  /// Work area (taskbar excluded) of the display containing the window,
+  /// in logical px. Returns null if screen info is unavailable.
+  static Future<Size?> workAreaSize({
+    required Offset windowPosition,
+  }) async {
+    try {
+      final displays = await screenRetriever.getAllDisplays();
+
+      // Pencerenin bulunduğu monitörü bul (çoklu monitör desteği)
+      Display display = await screenRetriever.getPrimaryDisplay();
+      for (final d in displays) {
+        final pos = d.visiblePosition;
+        final size = d.visibleSize ?? d.size;
+        if (pos != null &&
+            windowPosition.dx >= pos.dx &&
+            windowPosition.dx < pos.dx + size.width &&
+            windowPosition.dy >= pos.dy &&
+            windowPosition.dy < pos.dy + size.height) {
+          display = d;
+          break;
+        }
+      }
+
+      return display.visibleSize ?? display.size;
+    } catch (_) {
+      // Fallback: screen info unavailable → caller silently no-ops.
+      return null;
+    }
+  }
+
   /// Cascade: place new window to the right of the previous one.
   /// Falls to the next row (row start) when it would overflow the screen,
   /// and wraps to the top-left if the next row also overflows.