refactor: split shop_components.ex into 5 focused sub-modules

4,487-line monolith → 23-line facade + 5 modules:
- Base (inputs, buttons, cards)
- Layout (header, footer, mobile nav, shop_layout)
- Cart (drawer, items, order summary)
- Product (cards, gallery, variant selector, hero)
- Content (rich text, images, contact, reviews)

`use SimpleshopThemeWeb.ShopComponents` imports all sub-modules.
No single file over ~1,600 lines now.

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

lib/simpleshop_theme_web/components/layouts/shop.html.heex

@@ -1,2 +1,2 @@
-<SimpleshopThemeWeb.ShopComponents.shop_flash_group flash={@flash} />
+<.shop_flash_group flash={@flash} />
 {@inner_content}