Task 1: Fix stacked free-shipping pricing logic

[iamkarthik2004]

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[iamkarthik2004]

Completes all three OrbitStack challenge tasks, including the required AI usage documentation.

Task 1: Fix Pricing Logic

• Fixed stacked free-shipping logic for Pro users.
• Removed the incorrect addition of the standard shipping amount to the product discount.
• Added regression tests for stacked free-shipping benefits, code-only free shipping, and standard paid shipping.

Task 2: Optimize Dashboard Performance

• Removed the N+1 order-history query pattern.
• Loads orders, items, and products through a single Prisma relation query.
• Limits dashboard history to the last 30 days.
• Added a composite (userId, createdAt) index and reduced fetched fields to those used by the dashboard.

Task 3: Duplicate Order Feature

• Added POST /api/orders/[id]/duplicate.
• Enforces authentication and order ownership.
• Adds only active products with sufficient current stock to the cart and skips unavailable items.
• Added a Duplicate Order dashboard button with loading, success, partial-stock, and error feedback.

Documentation and Verification

• Updated README.md with Task 1, Task 2 and Task 3 implementation details.
• Completed AI_LOG.md.