⚡ Bolt: [performance improvement] Memoize cart context and component calculations

[Dev22603]

💡 What: Implemented useMemo for derived cart data computations (totalQuantity, totalPrice) in CartContext.jsx and Cart.jsx. Replaced inline reduce computation in Navbar.jsx with direct context value consumption.

🎯 Why: To prevent expensive array reduce operations from running unnecessarily on every render, especially on high-frequency components like Navbar that update on scrolling and user menu toggles.

📊 Impact: Reduces computational overhead by memoizing totals which are only calculated when cartItems changes. Removes redundant calculations across components that consume CartContext.

🔬 Measurement: Verifiable via React Profiler. Navbar and CartContext consumers no longer perform reduce calculations on unrelated state updates.

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PR created automatically by Jules for task 11316052509901355524 started by @Dev22603

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