feat: implement slide-out cart drawer

[Daksh7785]

resolves #162

Summary

Currently, clicking "Add to Cart" only updates a tiny counter in the Navbar. Users had no quick way to review the contents of their cart without navigating to a separate page, creating unnecessary friction during the shopping experience.

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Root Cause

The frontend state exclusively tracked book objects pushed to an array and did not support granular quantity management, and there was no intermediate UI (like a drawer) to visualize this state to the user without hard-routing.

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Solution

• Extracted a slide-out CartDrawer component from the main layout.
• Rewrote the global cart logic in App.jsx to group duplicate items and implement a quantity tracker.
• Supported decrement, increment, and strict removal logic.
• Built an overlay mechanism that traps body scroll and gracefully dismisses via mouse click, explicit button presses, or Keyboard Escape events.

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Files Changed

• bookshelf-frontend/src/App.jsx: State management augmented to handle array maps for quantity logic. Injected <CartDrawer />.
• bookshelf-frontend/src/components/CartDrawer.jsx (NEW): Core layout for the cart UI, rendering dynamic lists of books and calculating totals.
• bookshelf-frontend/src/components/CartDrawer.css (NEW): Standard UI layout matching established theme tokens.

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Testing

• Build passed
• Lint passed
• Tests passed
• Manual verification completed (Verified cart groupings, dynamic subtotal checks, and Escape key dismissal logic).

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Impact

• Breaking changes: No
• Performance impact: Lightweight conditional rendering via React.
• Security impact: None.
• Compatibility: Handled with scalable CSS flexbox properties suitable for mobile viewports.

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Checklist

• Issue solved
• Build passes
• Tests pass
• Lint passes
• No unrelated changes
• Ready for review

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