feat: add /collections/:slug routes with category filtering

- Add ShopLive.Collection LiveView for collection pages
- Add products_by_category/1 and category_by_slug/1 to PreviewData
- Support /collections/all for all products view
- Remove /products route and ShopLive.Products (replaced by collections)
- Update all links to use /collections/all instead of /products
- Update category nav to link to /collections/:slug
- Update PDP breadcrumb to link to specific collection

URL structure now follows Shopify convention:
- /collections/all - All products
- /collections/art-prints - Art Prints collection
- /collections/apparel - Apparel collection
- etc.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/simpleshop_theme_web/components/page_templates/pdp.html.heex

@@ -10,7 +10,7 @@
   <main id="main-content" class="max-w-7xl mx-auto px-4 sm:px-6 lg:px-8 py-8">
     <.breadcrumb items={[
       %{label: "Home", page: "home", href: "/"},
-      %{label: @product.category, page: "collection", href: "/products"},
+      %{label: @product.category, page: "collection", href: "/collections/#{@product.category |> String.downcase() |> String.replace(" ", "-")}"},
       %{label: @product.name, current: true}
     ]} mode={@mode} />