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/theme/preview_data.ex

@@ -114,6 +114,34 @@ defmodule SimpleshopTheme.Theme.PreviewData do
     end
   end
 
+  @doc """
+  Returns a category by its slug.
+
+  Returns nil if not found.
+  """
+  def category_by_slug(slug) do
+    Enum.find(categories(), fn cat -> cat.slug == slug end)
+  end
+
+  @doc """
+  Returns products filtered by category slug.
+
+  If slug is nil or "all", returns all products.
+  """
+  def products_by_category(nil), do: products()
+  def products_by_category("all"), do: products()
+
+  def products_by_category(slug) do
+    case category_by_slug(slug) do
+      nil ->
+        []
+
+      category ->
+        products()
+        |> Enum.filter(fn product -> product.category == category.name end)
+    end
+  end
+
   @doc """
   Checks if the shop has real products.