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/infrastructure/db.rs

@@ -282,6 +282,7 @@ impl Db {
             SyncAction::Delete => "Delete",
             SyncAction::Reorder => "Reorder",
             SyncAction::CreateList => "CreateList",
+            SyncAction::DeleteList => "DeleteList",
         };
         let conn = self.conn.lock().unwrap();
         conn.execute(
@@ -362,6 +363,7 @@ impl Db {
                     "\"Delete\"" | "Delete" => SyncAction::Delete,
                     "\"Reorder\"" | "Reorder" => SyncAction::Reorder,
                     "\"CreateList\"" | "CreateList" => SyncAction::CreateList,
+                    "\"DeleteList\"" | "DeleteList" => SyncAction::DeleteList,
                     _ => SyncAction::Update,
                 };
                 Ok(SyncQueueItem {