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>

test/simpleshop_theme_web/controllers/error_html_test.exs

@@ -4,11 +4,17 @@ defmodule SimpleshopThemeWeb.ErrorHTMLTest do
   # Bring render_to_string/4 for testing custom views
   import Phoenix.Template, only: [render_to_string: 4]
 
-  test "renders 404.html" do
-    assert render_to_string(SimpleshopThemeWeb.ErrorHTML, "404", "html", []) == "Not Found"
+  test "renders 404.html with themed page" do
+    html = render_to_string(SimpleshopThemeWeb.ErrorHTML, "404", "html", [])
+    assert html =~ "404"
+    assert html =~ "Page Not Found"
+    assert html =~ "shop-root"
   end
 
-  test "renders 500.html" do
-    assert render_to_string(SimpleshopThemeWeb.ErrorHTML, "500", "html", []) == "Internal Server Error"
+  test "renders 500.html with themed page" do
+    html = render_to_string(SimpleshopThemeWeb.ErrorHTML, "500", "html", [])
+    assert html =~ "500"
+    assert html =~ "Server Error"
+    assert html =~ "shop-root"
   end
 end