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/demo/theme.json

@@ -0,0 +1,12 @@
+{
+    "title": "demo",
+    "config": {
+        "default_template": "left_sidebar"
+    },
+    "template": {
+        "full_content": "full_content.twig",
+        "left_sidebar": "left_sidebar.twig",
+        "right_sidebar": "right_sidebar.twig",
+        "custom1": "custom1.twig"
+    }
+}