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-lock.json

@@ -9,6 +9,7 @@
       "version": "0.1.0",
       "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",
@@ -1489,6 +1490,15 @@
         "node": ">= 10"
       }
     },
+    "node_modules/@tauri-apps/plugin-dialog": {
+      "version": "2.6.0",
+      "resolved": "https://registry.npmjs.org/@tauri-apps/plugin-dialog/-/plugin-dialog-2.6.0.tgz",
+      "integrity": "sha512-q4Uq3eY87TdcYzXACiYSPhmpBA76shgmQswGkSVio4C82Sz2W4iehe9TnKYwbq7weHiL88Yw19XZm7v28+Micg==",
+      "license": "MIT OR Apache-2.0",
+      "dependencies": {
+        "@tauri-apps/api": "^2.8.0"
+      }
+    },
     "node_modules/@tauri-apps/plugin-opener": {
       "version": "2.5.3",
       "resolved": "https://registry.npmjs.org/@tauri-apps/plugin-opener/-/plugin-opener-2.5.3.tgz",