Offline queue: post now, sync later

A guestbook whose posts survive losing the network. With offline: { queue: true, persist: true }, an entry posted while disconnected queues in IndexedDB, survives a full tab reload, and replays exactly once on reconnect - the queue synthesizes an idempotency key per mutation and the server's live.idempotent wrapper dedups the replay.

Posting as Fluffy Mango (30b4b812)