Migrate background-sync off the deprecated utils/offline-queue (shared IndexedDB name)

[TortoiseWolfe]

Split out of #82, where it was mistakenly listed as a straight deletion.

Why it is not a deletion

src/utils/offline-queue.ts is marked @deprecated and superseded by src/lib/offline-queue/form-adapter.ts, but it is still live in production:

/contact  ->  ContactForm  ->  useWeb3Forms.ts:15  ->  utils/background-sync.ts:11  ->  utils/offline-queue.ts

background-sync.ts pulls getQueuedItems, removeFromQueue and updateRetryCount from it.

Why it needs care rather than an import swap

useWeb3Forms already straddles both systems — line 14 writes via addToQueue from @/lib/offline-queue, line 15 registers background sync, which reads through the old module.

Both implementations target the same IndexedDB database name, 'OfflineFormSubmissions' (src/utils/offline-queue.ts:21, src/lib/offline-queue/form-adapter.ts:37). So they are not two independent stores that can be swapped — they are two implementations pointed at one database, and whether their schemas and version numbers are equivalent is unverified.

Getting this wrong risks queued-but-unsent contact form submissions on real users' devices.

Work

• Diff the two schemas: object store names, key paths, indexes, version numbers, and the record shape each writes
• Decide whether a migration is needed or the stores are already compatible
• Point background-sync.ts at @/lib/offline-queue
• Test with a populated queue from the OLD implementation, not just an empty one — that is the case that can lose data
• Then delete src/utils/offline-queue.ts and its .browser.test.ts, and drop the vi.mock at src/hooks/useWeb3Forms.test.ts:23

Already done

The module-scope console.warn is now dev-only (5ef426b). It had been printing in the console of every visitor to /contact. The debt is no longer user-visible, which lowers the urgency but does not remove it.