feat: add session import/export functionality

- Add JSON export with full session metadata for backup/restore
- Add Markdown export with formatted conversation history
- Add import capability for previously exported sessions
- Add tauri-plugin-fs for file system operations
- Add export dropdown menu and import button to session history panel

package.json

@@ -26,6 +26,7 @@
     "@tauri-apps/api": "^2",
     "@tauri-apps/plugin-clipboard-manager": "^2.3.2",
     "@tauri-apps/plugin-dialog": "^2",
+    "@tauri-apps/plugin-fs": "^2.4.5",
     "@tauri-apps/plugin-notification": "^2",
     "@tauri-apps/plugin-opener": "^2",
     "@tauri-apps/plugin-os": "^2",