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

src-tauri/src/lib.rs

@@ -8,6 +8,7 @@ fn greet(name: &str) -> String {
 pub fn run() {
     tauri::Builder::default()
         .plugin(tauri_plugin_opener::init())
+        .plugin(tauri_plugin_dialog::init())
         .invoke_handler(tauri::generate_handler![greet])
         .run(tauri::generate_context!())
         .expect("error while running tauri application");