feat: add Cart page and themed error pages

- Add ShopLive.Cart at /cart using shared PageTemplates
- Update ErrorHTML to render fully themed 404/500 pages
- Add dev-only error preview routes at /dev/errors/404 and /dev/errors/500
- Update error page tests for themed output

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

lib/simpleshop_theme_web/live/shop_live/cart.ex

@@ -0,0 +1,68 @@
+defmodule SimpleshopThemeWeb.ShopLive.Cart do
+  use SimpleshopThemeWeb, :live_view
+
+  alias SimpleshopTheme.Settings
+  alias SimpleshopTheme.Media
+  alias SimpleshopTheme.Theme.{CSSCache, CSSGenerator, PreviewData}
+
+  @impl true
+  def mount(_params, _session, socket) do
+    theme_settings = Settings.get_theme_settings()
+
+    generated_css =
+      case CSSCache.get() do
+        {:ok, css} -> css
+        :miss ->
+          css = CSSGenerator.generate(theme_settings)
+          CSSCache.put(css)
+          css
+      end
+
+    logo_image = Media.get_logo()
+    header_image = Media.get_header()
+
+    # For now, use preview data for cart items
+    # In a real implementation, this would come from session/database
+    cart_page_items = PreviewData.cart_items()
+    cart_page_subtotal = Enum.reduce(cart_page_items, 0, fn item, acc ->
+      acc + item.product.price * item.quantity
+    end)
+
+    socket =
+      socket
+      |> assign(:page_title, "Cart")
+      |> assign(:theme_settings, theme_settings)
+      |> assign(:generated_css, generated_css)
+      |> assign(:logo_image, logo_image)
+      |> assign(:header_image, header_image)
+      |> assign(:cart_page_items, cart_page_items)
+      |> assign(:cart_page_subtotal, cart_page_subtotal)
+      |> assign(:mode, :shop)
+      |> assign(:cart_items, PreviewData.cart_drawer_items())
+      |> assign(:cart_count, length(cart_page_items))
+      |> assign(:cart_subtotal, format_subtotal(cart_page_subtotal))
+
+    {:ok, socket}
+  end
+
+  @impl true
+  def render(assigns) do
+    ~H"""
+    <SimpleshopThemeWeb.PageTemplates.cart
+      theme_settings={@theme_settings}
+      logo_image={@logo_image}
+      header_image={@header_image}
+      cart_page_items={@cart_page_items}
+      cart_page_subtotal={@cart_page_subtotal}
+      mode={@mode}
+      cart_items={@cart_items}
+      cart_count={@cart_count}
+      cart_subtotal={@cart_subtotal}
+    />
+    """
+  end
+
+  defp format_subtotal(subtotal_pence) do
+    "£#{Float.round(subtotal_pence / 100, 2)}"
+  end
+end