feat(dmg): use create-dmg for styled macOS DMG installer

Tauri's built-in DMG styling relies on AppleScript/Finder access which
silently fails on CI (tauri-apps/tauri#1731). Switch to create-dmg tool
which works on GitHub Actions macOS runners.

- Replace Tauri DMG with create-dmg: background image, icon positions,
  app-drop-link, codesign, hide-extension
- Regenerate background image at 2x Retina resolution (1320x800)
- Revert tauri.conf.json dmg config (ineffective on CI)
- Reorder steps: Prepare → Notarize DMG → Verify
- Notarize and Verify now use release-assets/ path for DMG

src-tauri/tauri.conf.json

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