Add IndexedDB sync queue with FIFO drain and OPFS blob store

[unforced]

Summary

Foundation for offline-capable CRUD in v0.2. Mutations issued offline are persisted in IndexedDB and drained in FIFO order when the vault is reachable.

• src/lib/sync/ — db (stores: pending autoIncrement seq, id_map, blob_path_map, blobs, meta), queue (enqueue + drain), engine (online/interval tick), blob-store (OPFS → IDB fallback), id-map (local-id + blob-ref resolution), storage-quota.
• src/providers/SyncProvider.tsx — opens the DB, builds the blob store, listens for online/offline, runs the engine, invalidates TanStack caches on drain.
• src/lib/vault/queries.ts — existing hooks now fall through to enqueue(...) when navigator.onLine === false. Same signatures; create returns an optimistic Note with a local-<uuid> id.

Design decisions worth calling out

• Local → remote ID mapping, not mutation-row routing. When create-note drains, we store {localId → serverId} in id_map. Subsequent update/delete rows in the queue call resolveNoteId(localId) before dispatching. Same pattern for blob uploads (blob:<blobId> → storage path). This keeps the mutation rows themselves small and independent, and means UI code never has to be told "your note id changed."
• FIFO via autoincrement seq. Queue depth is small (one user, thinking-pace mutations), so the cursor filters vault + status in-app rather than maintaining a compound index.
• ArrayBuffer, not Blob, across the blob-store interface. Both jsdom and fake-indexeddb mangle Blob on structured clone. Callers convert via the exported blobToArrayBuffer() helper.
• OPFS mimeType in sidecar files. OPFS has no metadata channel of its own — we write <blobId>.meta alongside each payload.
• Error classification: VaultAuthError → halt drain + meta["auth-halted"] marker; VaultConflictError → stash as needs-human, continue past it; VaultNotFoundError → drop the row; anything else → exponential backoff (2^attempts seconds, capped at 10 min).
• Vault isolation. A drain only touches rows for the active vault. Other vaults' queues are left alone.

Six mutation kinds

create-note, update-note, delete-note, upload-attachment, link-attachment, delete-attachment. All resolve local ids + blob refs before hitting the network.

Out of scope (per PR #2 spec)

UI for queue status, dedicated conflict-resolution UI, voice capture wiring, scribe. This is plumbing only — the existing UI simply stops failing when offline.

Gates

• bun run lint ✅
• bun run typecheck ✅
• bun run test --run — 231/231 passing (39 new sync tests)
• bun run build ✅

Test plan

• DevTools → Network → Offline, create a note → optimistic note appears with local-* id; reload → note list empty (expected, since the queue lives in IDB and drains later).
• Back online, wait for next tick (or fire an online event) → note appears with server id.
• Offline → update / delete of an existing server note → queued; drains when online.
• Auth halt: invalidate token, mutate offline → back online → drain halts + meta["auth-halted"] set.
• Restart resilience: enqueue offline, hard reload tab, confirm pending count persists.

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