Improve checkout process with inventory validation and error handling

[seer-by-sentry]

Fixes REACT-60M. The issue was that: Backend inventory validation failed due to an UnboundLocalError for 'quantities', causing the fetch request to fail and resulting in a frontend TypeError.

• Added deep copy to avoid modifying the global inventory directly during order processing.
• Added validation to check if the product ID exists in the inventory.
• Implemented JSON parsing error handling for the checkout route.
• Added validation to ensure the request body contains a 'cart' key and that it is a list.

This fix was generated by Seer in Sentry, triggered by naru tsuka. 👁️ Run ID: 176196

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