[ADD] website_sale_lazy_loading: load the /shop listing on demand

The shop pages products: every page is a full reload that throws away the
grid the visitor was reading. This appends the next page to the grid
instead, on scroll or on a "Load more products" click, per website.

The controller does not duplicate /shop. The new /shop/lazy_products calls
shop() and renders the cards out of the qcontext it prepared, so search,
categories, attributes, tags, price filter, sort order and pricelists are
supported by construction -- and so are the values other modules add to the
listing, the wishlist state among them. Out of range pages answer empty
rather than the last page again, which portal.pager would otherwise clamp
to and the frontend would append as duplicates.

The product loop of website_sale.products is replaced by a call to a shared
template, so the first page and the appended ones are the same markup: a
ribbon, a price or a button another module adds to products_item shows up
on every card, not only on the ones the initial render produced.

Progressive enhancement throughout: the first page and the pager are still
what the standard controller renders, and the pager is only hidden once the
widget is running. Without JavaScript -- and for crawlers -- the shop is
exactly what it is without this module. The page size is the shop layout's
"Products per page", the value the pager already uses, so there is nothing
to keep in sync.

The frontend takes no decision about what to show: the server sends the
mode, the URL of the listing on screen and each page of cards. It observes
a block below the grid rather than listening to scroll, restarts the public
widgets on the appended cards, and keeps a button as the fallback for a
failed request or a browser without IntersectionObserver.

Tests cover the block rendering per mode, the filters travelling in the
AJAX URL, and the endpoint on a next page, the last page, an out of range
page, a broken page number and an unknown category.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

website_sale_lazy_loading/readme/DESCRIPTION.rst

@@ -0,0 +1,20 @@
+This module loads the products of the eCommerce shop (``/shop``) **on demand**
+instead of paginating them: the visitor keeps the products already on screen
+and the next page is appended to the grid, either when they scroll to the
+bottom of the listing (infinite scroll) or when they click a *Load more
+products* button.
+
+It is the same idea as the lazy loading of ``website_sale_aplicoop``
+(``/eskaera``), rebuilt for the standard shop:
+
+* The page size is the **Products per page** of the shop layout, the very same
+  value the standard pager uses, so nothing has to be configured twice.
+* The product cards come from the standard ``website_sale.products_item``
+  template, so ribbons, prices, wishlist or comparison buttons added by other
+  modules are rendered exactly as on the first page.
+* Search, categories, attributes, tags, price filter and sort order are kept:
+  the appended pages are the pages of the listing currently on screen.
+
+The first page and the pager are still rendered by the standard controller, so
+visitors without JavaScript — and search engine crawlers — keep a listing they
+can navigate page by page.