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/Cargo.toml

@@ -22,4 +22,5 @@ tauri = { version = "2", features = [] }
 tauri-plugin-opener = "2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
+tauri-plugin-dialog = "2.6.0"