Race condition in FileStorage leads to data loss

The `FileStorage` implementation in `packages/linejs/base/storage/file.ts` is vulnerable to race conditions. The `set` and `delete` methods perform asynchronous read-modify-write operations without any locking mechanism. Concurrent calls (e.g., during rapid token refreshes) can cause data to be overwritten by stale state, leading to loss of auth sessions or other critical data.
