Fix pricing, optimize dashboard, and add duplicate orders

[rayhq]

Summary

Completed all three OrbitStack coding challenge tasks.

Task 1 — Pricing Logic

• Fixed double-counting of free-shipping discounts.
• Preserved existing discount behavior.
• Prevented incorrect shipping discount calculations.

Task 2 — Dashboard Performance

• Removed the N+1 order/order-item query pattern.
• Optimized Prisma order fetching using relations.
• Limited dashboard order history to the required 30-day period.

Task 3 — Duplicate Order

• Added POST /api/orders/[id]/duplicate.
• Added authentication and ownership validation.
• Skips inactive/out-of-stock products.
• Handles available stock quantities.
• Added Duplicate Order UI to dashboard orders.
• Integrated with the existing Zustand cart store.

Validation

• npm run test — 11 tests passed
• npx tsc --noEmit — passed
• npm run build — passed

Documentation

• Updated AI_LOG.md with AI usage and development workflow.