CMS 2.0 - Theme engine, logging, admin improvements

Major changes:
- New ThemeManager with Twig templating and SCSS compilation
- Dynamic themes system (themes/default, themes/demo)
- LogManager with SQLite storage and syslog forwarding
- RequestLogger with static helper methods
- Admin UI overhaul (Bootstrap 5, dark mode)
- Admin config page with logging and theme settings
- Admin logs page with filters and search
- Removed legacy Mustache templates
- Removed test plugin and theme
- Composer dependencies: Twig, scssphp, CommonMark, MaxMind GeoIP

admin/templates/layout.php

@@ -171,6 +171,11 @@ $layoutSidebarColor = $layoutThemeConfig['header_color'] ?? '#0a369d';
             case 'theme':
                 require __DIR__ . '/pages/theme.php';
                 break;
+
+            case 'theme-new':
+                require __DIR__ . '/pages/theme-new.php';
+                break;
+
             case 'plugins':
                 require __DIR__ . '/pages/plugins.php';
                 break;