[19.0][ADD] website_sale_product_sticky_add_to_cart: sticky add-to-cart bar on product page

[idris-domatix]

Description of the issue/feature

Add a sticky Add to Cart bar to the e-commerce product page. While the
visitor scrolls down, a fixed bar slides up from the bottom showing the
product thumbnail, its name, the current price and an Add to Cart button
that triggers the standard website_sale add-to-cart flow.

The bar is rendered server-side (hidden with a CSS transform so it never
affects page scrolling) and an interaction:

• toggles its visibility past a scroll threshold;
• keeps the price in sync with the currently selected variant via a
  MutationObserver on the standard page price;
• forwards the button click to the regular #add_to_cart button.

The bar can be enabled/disabled per website from the Customize menu of the
product page.

This is a fresh, minimal implementation following OCA conventions; the same
behaviour has been running in production on several Odoo 19 shops.

@OCA/e-commerce-maintainers please review.

[idris-domatix]

Ready for review.

This module adds a sticky Add to Cart bar to the e-commerce product page: while scrolling, a fixed bar slides up showing the product thumbnail, name, current price and an Add to Cart button that triggers the standard website_sale flow. It is implemented with a server-side template inheritance (customizable from the product page Customize menu) plus a frontend interaction that toggles the bar on scroll and keeps the price in sync with the selected variant.

All CI checks are green (pre-commit, test with Odoo/OCB, runboat, codecov). The behaviour has also been validated on a live Odoo 19 shop.

[idris-domatix]

Friendly ping for review. This is a new 19.0 module (sticky add-to-cart bar) and CI is green. @OCA/e-commerce-maintainers could someone take a look when you have a moment?