Can I vibe code SellUp?

Build a custom Shopify upsell application using standard Shopify Theme App Extensions (Liquid + Web Components/vanilla JS) and optional Remix app for configuration.

1. ARCHITECTURE & DATA MODEL:
- Store cross-sell mappings either via Shopify Product Metafields (`namespace: cross_sell`, `key: offer_variant_id`, `type: variant_reference`) or inside a lightweight SQLite/PostgreSQL schema with fields: trigger_product_id, offer_variant_id, custom_title, custom_discount_percent, offer_type ('modal' or 'embedded').

2. FRONTEND ENGINE (THEME APP EXTENSION):
- Create a App Block component injectable on Product Pages and Cart Drawer.
- Listen for add-to-cart events intercepting submit on forms matching `action='/cart/add'` or standard Shopify ajax cart events.
- When triggered, evaluate if an offer exists for the current product variant ID.
- If an offer exists, present a clean, unstyled modal dialog (using native `<dialog>` element) showing: target image, custom title, price comparison, variant selector dropdown, and 'Add to Order' button.

3. CART API INTEGRATION:
- When 'Add to Order' is clicked, send a POST request to `/cart/add.js` containing the offer variant ID.
- Support applying line-item properties if a discount flag is enabled.
- Automatically trigger standard Shopify theme cart refresh events (e.g. dispatch `cart:refresh` or trigger theme-specific cart drawer opening).
- Handle inventory validation: hide the offer if variant `available` status is false or inventory quantity is zero.

4. SESSION & FAILURE MODES:
- Use `sessionStorage` to prevent showing the exact same upsell modal twice within the same visitor session if dismissed.
- Gracefully fall back: if network fails or JS errors out, proceed with normal standard cart submission without blocking user checkout.

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