Feat/product variants

[nikhildhamdhere15]

Description

Closes #260

Implements a full-stack data pipeline to support multi-attribute product variants (Size and Color) with independent pricing and stock tracking.

Changes Made

• Database: Embedded variantSchema into product.model.js and added variantId tracking to cart.model.js.
• Backend: Updated product and cart controllers to handle stock validation and variant filtering.
• Frontend: Rewrote ProductPage.jsx with dynamic Chakra UI dropdowns for interactive variant filtering.
• State Management: Overhauled the Zustand cart store to cleanly pass variantId payloads to the API.

Verification

• Tested locally; verified dynamic UI price/stock updates and backend payload tracking.

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