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

composer.json

@@ -3,6 +3,8 @@
         "mustache/mustache": "^3.0",
         "league/commonmark": "^2.7",
         "php-mqtt/client": "^2.0",
-        "geoip2/geoip2": "^2.13"
+        "geoip2/geoip2": "^2.13",
+        "twig/twig": "^3.28",
+        "scssphp/scssphp": "^2.1"
     }
 }