feat(backorders): BACK-472 render backorder quantity prompt for complex products on PDP

[bc-apostoliuk]

Jira: BACK-472

What/Why?

Screen.Recording.2026-03-02.at.14.05.42.mov

The "quantity on backorder" prompt on the PDP only worked for simple products. For complex products (with variants), availableOnHand and availableForBackorder were injected once at page load and never updated when a different variant was selected, causing incorrect backorder calculations.

This PR:

• Injects availableToSell, showBackorderAvailabilityPrompt, and the i18n backorder message (products.quantity_backordered) into the JS context
• Adds updateBackorderContext() to refresh variant-level inventory values (availableOnHand, availableForBackorder, availableToSell) from the optionChange API response on every variant selection
• Falls back to data.stock (and the displayed stock DOM value) when available_on_hand is not present in the API response
• Recalculates the backorder message inside updateView() after every variant change
• Replaces the hardcoded backorder text with the i18n string
• Respects the merchant's showBackorderAvailabilityPrompt setting
• Preserves backward compatibility for simple products

Rollout/Rollback

Standard deployment. Rollback by reverting the PR if needed.

Testing

1. Simple product with backorder enabled:

   • Navigate to PDP of a simple product with stock tracking and backorder enabled
   • Set quantity > on-hand stock → verify "{X} will be backordered" message appears with correct count
   • Set quantity <= on-hand stock → verify message hides

2. Complex product (with variants) with backorder enabled:

   • Navigate to PDP of a product with multiple variants, stock tracking, and backorder enabled
   • Select a variant → verify stock level updates
   • Set quantity > on-hand stock for selected variant → verify backorder message appears with correct count
   • Switch to a different variant with different stock → verify backorder message recalculates correctly
   • Set quantity <= on-hand stock → verify message hides

3. Merchant settings:

   • With "Quantity on backorder" display disabled → verify message never appears
   • With inventory display set to "Don't Show" → verify stock container and backorder message are hidden

4. Edge cases:

   • Quantity exactly equal to on-hand → no backorder message
   • Quantity exceeding availableToSell → no backorder message (OOS)

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