feat: add custom multi-folder picker

Replace native add-folder dialogs with an in-app folder picker that supports collecting folders from multiple locations before adding them together.

Add backend directory listing and batch add commands with duplicate skipping, plus store actions and a themed picker UI with a dedicated folders-to-add panel.

src-tauri/src/lib.rs

@@ -128,6 +128,8 @@ pub fn run() {
         })
         .invoke_handler(tauri::generate_handler![
             commands::add_folder,
+            commands::add_folders,
+            commands::list_directories,
             commands::get_folders,
             commands::reorder_folders,
             commands::get_background_job_progress,