Offline PWA: IndexedDB outbox + background sync

[rec3141]

Part of #1.

Persist captured samples offline and sync on reconnect, using the already-reserved client_id / local_created_at / sync_version columns on samples.

Scope

• IndexedDB outbox: each completed sample → a row with client_id = crypto.randomUUID() + local_created_at. Photos stage as Blobs.
• Background Sync (with an on-load flush fallback) to POST queued sites first, then samples (POST /api/samples), then photos.
• Idempotent dedup keyed on client_id so a retried POST doesn't double-insert.
• Surface queue state in the UI (N pending / synced / failed).

Notes

• sync_log table exists but is currently never written — decide whether sync should populate it.
• Offline site creation needs a client id too — see the site sub-wizard issue.

[rec3141]

Implemented on feat/offline-pwa (src/lib/offline/outbox.ts + wizard/SiteWizard integration). Typecheck + build are clean, but the offline runtime can't be exercised by either — here's the on-device manual test checklist before merge:

1. Install the PWA on a phone (Add to Home Screen). Confirm src/service-worker.js registers (DevTools → Application → Service Workers).
2. Online capture: complete a sample via /samples/wizard → row appears in /samples; photos + captions attached.
3. Offline site + sample: enable airplane mode → create a new site with GPS in the sub-wizard, then a sample referencing it → both show "saved offline"; the pending badge shows counts.
4. Reconnect: disable airplane mode → the on-online flush drains the queue. Confirm the site lands before the sample and the sample's site_id resolves (client-id-as-PK).
5. Idempotency: kill the network mid-sync after a sample POST succeeds but before the row is removed; reconnect → the duplicate-id retry is swallowed (no double row, queue clears).
6. Rejected record: force a validation error offline (e.g. blank required) → it should surface as failed/kept, not silently lost.
7. Cold offline load: visit the wizard online once, then fully offline → the form still renders (app-shell cache, Offline PWA: service worker + offline app shell #2).

Notes / known limitations to weigh in review:

• HTTP rejections are recorded on the queued row but not auto-retried (avoids dup storms); there's no UI yet to edit/delete a rejected row — only the "Sync now" retry.
• sync_log is still not written (the audit-table decision from the design doc is open).

[rec3141]

Fixed in commit 6f01f87: the flush dedup no longer infers success from a bare 409 (which a UNIQUE name collision also returns, silently dropping the capture). It now GETs the row by clientId via existsOnServer() and only drops the queue entry when our own row is confirmed present; a genuine collision is kept as a rejection. This removed looksDuplicate() and its empty-string asymmetry.