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

themes/default/full_content.twig

@@ -0,0 +1,11 @@
+{% extends 'base.twig' %}
+
+{% block content %}
+<div class="container">
+    <section id="site-content" class="col-12">
+        <div class="content-wrapper p-4">
+            {{ content|raw }}
+        </div>
+    </section>
+</div>
+{% endblock %}