[Feature] Persist carts and draft orders in SQLite

[moakilodash]

Problem

All cart data is currently stored in memory and is lost when the app restarts. This prevents users from resuming shopping sessions.

Proposed Change

Add a simple SQLite persistence layer that stores cart snapshots and draft orders. Cart state should survive app restarts without affecting the existing in-memory state management or UI layer.

Acceptance Criteria

• Cart state survives app restart
• Unit tests cover create, update, and clear cart paths
• Persistence layer is isolated from Textual widget code (service layer)
• Schema supports future order and inventory tracking
• No breaking changes to existing cart or store APIs

Related

See ROADMAP.md Phase 3 - Local Checkout and Persistence

Contributor Lane: Checkout & Persistence (Issue 3/2)
Difficulty: Medium
Related Phase: Phase 3 - Local Checkout and Persistence