Optional linked products offered per product. On a product page the customer can tick one or more add-ons (checkbox + quantity); each is added to the cart as its own line item, inheriting the parent's hub and booking dates — no re-entry of dates required.

• Module: product-addons/product-addons.php
• Status: Done, should work
• Stock impact: none to existing code — add-ons flow through the standard per-item stock/reservation/deduction paths, same as other proucts

1. Edit a product. In the Addons box, pick the products to offer as add-ons (field addon_products).
2. Each add-on product must be a plain WooCommerce product with a recipe (recette_composants) — not a PluginHive-bookable product. Its per-hub stock comes from the recipe, like any product.

1. On the product page, under the add-to-cart area, an add-ons section lists each configured add-on with its price.
2. Add-ons are greyed out until a hub with stock is selected. Availability is computed per hub and is reservation-aware.
3. Ticking an add-on reveals a quantity input, capped at the quantity available at the chosen hub.
4. On add-to-cart, every ticked add-on is added as its own cart line, carrying the parent's hub. The parent's booking dates are copied onto the add-on at checkout.

Mirrors the existing hub_id propagation chain.

1. Edit product ACF addon_products
2. → Product page UI builds a per-hub availability map; JS greys out hubs with no stock
3. → On the parent's woocommerce_add_to_cart, read $_POST['mobil_addons'] and add each ticked add-on as its own line with hub_id + parent link in cart_item_data
4. → reserve_components_on_add_to_cart reserves the add-on's composants (existing system)
5. → Cart display via woocommerce_get_item_data (“Add-on for: …”)
6. → Checkout: hub_id copied to the order item (existing); parent booking dates copied onto the add-on
7. → decrementer_stock_avec_composants_auto deducts stock per item (existing)

The feature was designed to not touch any stock code. Add-ons are ordinary separate line items, so every existing hook treats them like any product.

• Add-on products must be plain products with recette_composants. An add-on without a recipe is sold with no stock control (reservation/deduction skip it).
• Add-ons are independent of the parent in the cart: removing the parent does not remove its add-ons (by design).