Urgent E-commerce Site Fix

I have a nearly finished e-commerce store that my brother built with a custom, hand-coded stack (HTML, CSS and JavaScript—no WordPress, Shopify or Magento). Before he could push it live he hit a launch error on his local server, then had to step away for health reasons. Our opening date is looming, so I need someone to step in right away, diagnose whatever is blocking the build, and get the site running smoothly.

You can grab the full project archive here for a first look:
https://mega.nz/file/iFlUXCzI#zCcDLsLXMI7lMwutMKBdDO7JEAHqto3ROqQNClrsXns

What I need from you:
• Fix the current build/launch error so the site loads without issues locally.
• Clean up any dependencies or path problems you find while testing.
• Confirm all store functions—product listings, cart, checkout, payment hooks—work end-to-end.
• Hand over a ready-to-deploy package (or, if you prefer, help me push it to my hosting and walk me through a quick smoke test).

Acceptance criteria: the site compiles, serves and handles a full test purchase flow on my machine with no console errors.

If you’re comfortable jumping into a pure HTML/CSS/JS codebase and can turn this around fast, let’s talk and get the store open on time.