feat(desktop): offline-first via Turso Sync in the single-process host

[rickylabs]

Part of #327

Handle: #E5 · Milestone: 0.0.1-beta.8 · Depends on: #E4.

Scope — offline-first via Turso Sync in the single-process host

Turso Sync pull()/push() (default last-push-wins; transform hook for custom merges) in the single-process host; offline-capable operation, opportunistic reconcile on connectivity.

Acceptance

• App operates fully disconnected; reconciles on reconnect; conflict policy documented.
• Offline-first claim is technically true only in option (c) (single local writer) — do not market it for (a)/(b) (turso-sync-offline-first.md).

Design source: design/E-desktop/epic-and-issues.md (#E5).

[rickylabs]

Dependency edge (DAG second pass): Depends on #454 (E4).