add quantity controls to cart drawer via shared CartHook

Move increment/decrement handlers from Cart LiveView into CartHook so
they work from any page's drawer. Enable show_quantity_controls on the
drawer's cart_item_row. Scope cart tests to #main-content to avoid
duplicate button matches.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/simpleshop_theme_web/components/shop_components/cart.ex

@@ -105,7 +105,7 @@ defmodule SimpleshopThemeWeb.ShopComponents.Cart do
           <ul role="list" aria-label="Cart items" style="list-style: none; margin: 0; padding: 0;">
             <%= for item <- @cart_items do %>
               <li style="border-bottom: 1px solid var(--t-border-default);">
-                <.cart_item_row item={item} size={:compact} mode={@mode} />
+                <.cart_item_row item={item} size={:compact} show_quantity_controls mode={@mode} />
               </li>
             <% end %>
           </ul>