improve error pages: minimal version when site not live

Show a lightweight error page using admin.css when the shop isn't
live yet, avoiding broken theme dependencies. Also tidied up copy
to sentence case and shorter descriptions.

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

test/berrypod_web/controllers/error_html_test.exs

@@ -4,17 +4,15 @@ defmodule BerrypodWeb.ErrorHTMLTest do
   # Bring render_to_string/4 for testing custom views
   import Phoenix.Template, only: [render_to_string: 4]
 
-  test "renders 404.html with themed page" do
+  test "renders 404.html" do
     html = render_to_string(BerrypodWeb.ErrorHTML, "404", "html", [])
     assert html =~ "404"
-    assert html =~ "Page Not Found"
-    assert html =~ "shop-root"
+    assert html =~ "Page not found"
   end
 
-  test "renders 500.html with themed page" do
+  test "renders 500.html" do
     html = render_to_string(BerrypodWeb.ErrorHTML, "500", "html", [])
     assert html =~ "500"
-    assert html =~ "Server Error"
-    assert html =~ "shop-root"
+    assert html =~ "Server error"
   end
 end