fix CSS nesting warnings and test type warnings

Replace BEM-style &--modifier nesting (unsupported in native CSS) with
&.class-modifier in shop/components.css and page_renderer.ex. Fix
Elixir 1.19 type warnings comparing lists with != [].

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

test/berrypod/theme/preview_data_test.exs

@@ -76,7 +76,7 @@ defmodule Berrypod.Theme.PreviewDataTest do
       cart_items = PreviewData.cart_items()
 
       assert is_list(cart_items)
-      assert cart_items != []
+      assert length(cart_items) > 0
     end
 
     test "each cart item has required fields" do
@@ -116,7 +116,7 @@ defmodule Berrypod.Theme.PreviewDataTest do
       testimonials = PreviewData.testimonials()
 
       assert is_list(testimonials)
-      assert testimonials != []
+      assert length(testimonials) > 0
     end
 
     test "each testimonial has required fields" do