feat(dmg): customize macOS DMG installer appearance

- Add DMG background image with drag-to-install arrow guide
- Configure window size (660x400), app and Applications icon positions
- Center icons horizontally with visual arrow between them

src-tauri/tauri.conf.json

@@ -50,7 +50,22 @@
       }
     },
     "macOS": {
-      "minimumSystemVersion": "12.0"
+      "minimumSystemVersion": "12.0",
+      "dmg": {
+        "background": "icons/dmg-background.png",
+        "windowSize": {
+          "width": 660,
+          "height": 400
+        },
+        "appPosition": {
+          "x": 180,
+          "y": 220
+        },
+        "applicationFolderPosition": {
+          "x": 480,
+          "y": 220
+        }
+      }
     }
   },
   "plugins": {