Fix: Move useOptimistic to provider level for consistent cart state

[francomay3]

Fix: Move useOptimistic to provider level for consistent cart state

Background:
Discovered a bug where there was noticeable lag between adding products to cart and seeing the cart update. Initially confusing because useOptimistic was being used throughout the codebase, but the issue was that each useCart() hook call was creating its own optimistic cart instance.

Problem:
Cart updates had lag because useOptimistic was being created in each useCart() hook call, resulting in multiple optimistic cart instances. Components using the cart outside of the hook were seeing the actual cart (with server delay) while hook consumers saw the optimistic cart, causing inconsistent UI updates.

Solution:
Moved useOptimistic from the useCart hook to the CartProvider component, creating a single shared optimistic cart instance that all components can access through the context.

Result:
All cart operations now update instantly across the entire application, eliminating the lag between adding items and seeing them in the cart UI.

Files changed:
components/cart/cart-context.tsx

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