125Implement an offline-first data synchronization queue using IndexedDB

A field-sales app must keep working on a train with no signal: every change is applied locally at once and synced when the network returns, even if the tab was closed in between.

Implement createSyncQueue({ store, send, online = true, retryDelay = (attempt) => 1000 * 2 ** attempt }):

• store is a persistence adapter with async getAll(), put(record) and delete(id). In the browser it is backed by IndexedDB (see the adapter in the solution); in tests it is in memory.
• send(op) delivers one operation to the server. It rejects with a TypeError for network problems (retryable), or with an error that has a 4xx status for operations the server will never accept.

The queue returns { ready, enqueue(op, { coalesceKey }), flush(), setOnline(online), pending(), failed() }:

• ready is a promise that resolves after previously stored records are loaded (they survive reloads).
• enqueue persists { id, op, attempts: 0, status: "pending", coalesceKey } before resolving, then starts a flush if online. If coalesceKey is given and a pending record with the same key exists, replace its op instead of adding a new one (for example, ten edits to the same field send only the latest).
• flush() sends pending records in order, one at a time. On success, delete the record. On a network error, increase attempts, stop flushing (keeping the order) and retry after retryDelay(attempts - 1). On a 4xx error, mark the record "failed" (a dead letter for the UI to show) and continue with the next one. Only one flush runs at a time; calling flush() during a flush returns the same promise.
• setOnline(true) starts a flush; setOnline(false) stops starting new sends.
• pending() and failed() return the records in order.