feat: add Theme LiveView with preset switching

Implement basic theme editor interface with live preview:
- ThemeLive.Index LiveView with mount and event handlers
- Two-column layout: controls sidebar + preview area
- Display all 9 presets as clickable buttons
- Apply preset and regenerate CSS on click
- Show current theme settings (mood, typography, shape, density, color)
- Preview page switcher (7 pages: home, collection, product, cart, about, contact, 404)
- Inline <style> tag with generated CSS for instant preview
- Basic preview frame showing theme variables in action
- Authentication required via :require_authenticated_user pipeline
- Theme navigation link added to user menu
- 9 comprehensive LiveView tests

All tests passing (197 total).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

lib/simpleshop_theme_web/components/layouts/root.html.heex

@@ -36,6 +36,9 @@
         <li>
           {@current_scope.user.email}
         </li>
+        <li>
+          <.link href={~p"/admin/theme"}>Theme</.link>
+        </li>
         <li>
           <.link href={~p"/users/settings"}>Settings</.link>
         </li>