refactor: consolidate CSS to use .themed class with native nesting

- Add .themed class as shared selector for both shop and preview
- Move visual/behavioral styles from .preview-frame to .themed
- Keep .preview-frame only for CSS variable switching (editor live preview)
- Update CSSGenerator to target .themed instead of .shop-root
- Refactor CSS files to use native CSS nesting syntax
- Update tests to reflect new class structure

This improves maintainability by:
- Eliminating duplicate selectors (.shop-root + .preview-frame)
- Using modern CSS nesting (94%+ browser support)
- Clear separation: .preview-frame = vars, .themed = styles

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

test/simpleshop_theme/theme/css_generator_test.exs

@@ -10,10 +10,16 @@ defmodule SimpleshopTheme.Theme.CSSGeneratorTest do
       css = CSSGenerator.generate(settings)
 
       assert is_binary(css)
-      assert css =~ ".preview-frame, .shop-root"
+      # CSS targets .themed (used by both shop and preview)
+      assert css =~ ".themed {"
       assert css =~ "--t-accent-h:"
       assert css =~ "--t-accent-s:"
       assert css =~ "--t-accent-l:"
+      # Should include all theme token categories
+      assert css =~ "--t-surface-base:"
+      assert css =~ "--t-font-heading:"
+      assert css =~ "--t-radius-sm:"
+      assert css =~ "--t-density:"
     end
 
     test "converts hex colors to HSL" do