Fix PDF export with Tauri native dialogs and proper margins

- Installed @tauri-apps/plugin-dialog for native dialogs
- Added tauri-plugin-dialog to Rust dependencies
- Registered dialog plugin in Tauri app initialization
- Replaced web alert() with Tauri message() dialog
- Success dialog shows filename and download location
- Error dialog shows if export fails
- Added 20mm margins on all sides of PDF pages
- Content width adjusted to 170mm (210mm - 40mm margins)
- Multi-page support respects margins on all pages
- Native dialogs work properly in Tauri app

package.json

@@ -11,6 +11,7 @@
   },
   "dependencies": {
     "@tauri-apps/api": "^2",
+    "@tauri-apps/plugin-dialog": "^2.6.0",
     "@tauri-apps/plugin-opener": "^2",
     "@tiptap/extension-strike": "^2.27.2",
     "@tiptap/extension-underline": "^2.27.2",