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>

lib/simpleshop_theme_web/live/theme_live/index.html.heex

@@ -888,7 +888,7 @@
         </div>
 
         <!-- Preview Frame -->
-        <div class="preview-frame bg-white overflow-auto flex-1 rounded-b-lg border border-t-0 border-base-content/20"
+        <div class="themed preview-frame bg-white overflow-auto flex-1 rounded-b-lg border border-t-0 border-base-content/20"
              data-mood={@theme_settings.mood}
              data-typography={@theme_settings.typography}
              data-shape={@theme_settings.shape}