Fix list deletion sync: SyncAction::DeleteList

- Added DeleteList variant to SyncAction enum
- Added ApiClient::delete_list() calling DELETE /users/@me/lists/{id}
- List deletion uses DeleteList action (not Delete/delete_task)
- Sync engine handles DeleteList calling api.delete_list()

src/app.rs

@@ -739,12 +739,12 @@ impl App {
                             if self.selected_list < self.lists.len() {
                                 let list_id = self.lists[self.selected_list].id.clone();
                                 self.db.delete_list(&list_id).ok();
-                                self.db.push_sync(
-                                    SyncAction::Delete,
-                                    &list_id,
-                                    &list_id,
-                                    "",
-                                ).ok();
+                        self.db.push_sync(
+                            SyncAction::DeleteList,
+                            &list_id,
+                            &list_id,
+                            "",
+                        ).ok();
                                 self.trigger_sync();
                                 self.load_lists();
                                 if self.selected_list >= self.lists.len() {