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/partials/navigation.twig

@@ -0,0 +1,17 @@
+<nav class="navigation-section" role="navigation" aria-label="Main navigation">
+    <h2 class="sr-only">Site Navigation</h2>
+    <div class="container-fluid">
+        <div class="row align-items-center">
+            <div class="col">
+                <ul class="nav nav-tabs flex-wrap" role="menubar">
+                    <li class="nav-item" role="none">
+                        <a class="nav-link {{ home_active_class }}" href="/{{ current_lang }}" role="menuitem" {% if is_homepage %}aria-current="page"{% endif %}>
+                            <i class="bi bi-house" aria-hidden="true"></i> {{ homepage_title }}
+                        </a>
+                    </li>
+                    {{ menu|raw }}
+                </ul>
+            </div>
+        </div>
+    </div>
+</nav>