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

src-tauri/src/lib.rs

@@ -39,6 +39,7 @@ pub fn run() {
         .plugin(tauri_plugin_os::init())
         .plugin(tauri_plugin_http::init())
         .plugin(tauri_plugin_clipboard_manager::init())
+        .plugin(tauri_plugin_fs::init())
         .manage(bridge_manager.clone())
         .manage(temp_manager.clone())
         .setup(move |app| {