Fix duplicate order creation during checkout submission

[AanyaJain0811]

Summary

Rapid or repeated clicks on "Place Order" during checkout could fire multiple POST /orders requests before the first one resolved, since nothing blocked re-entry into placeOrder() while it was awaiting the API call. This created duplicate orders and triggered sendWhatsAppFinal() (and its WhatsApp window) more than once per checkout.

Changes Made

• Added a module-level isSubmitting flag, checked at the top of placeOrder(). Any call made while a request is already in flight returns immediately, so this is the actual fix — not just a UI nicety.
• Disabled the "Place Order" button and swapped its label to "Processing..." for the duration of the request, giving the user clear feedback that their click registered.
• Wrapped the request in try/catch/finally, so isSubmitting and the button's disabled state/label always reset — on success, on a handled failure (data.success === false), and on a thrown/network error alike.

Impact

• Eliminates duplicate order creation from double-clicks or repeated taps on slow connections.
• Prevents multiple WhatsApp windows from opening per checkout.
• No behavior change for the normal single-click flow; the button simply locks for the brief duration of the request and unlocks automatically regardless of outcome.

Fixes #908

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