System plugin support: admin menu items, admin routes, API integration

- PluginManager: getAdminMenuItems(), handleAdminRoute(), getPluginType()
- admin.php: load PluginManager, pass plugin_admin_menu to Twig
- admin.php: route to plugin admin pages via handleAdminRoute()
- admin.twig: show 'Plugins' sidebar section for system plugins
- plugins-new.twig: choose content or system plugin type
- handlePluginsNew: generate proper template based on type (content/system)
- plugin-admin.twig: renders plugin output in admin layout
- GeoIPInfo: example system plugin (admin page with GeoIP info)
- System plugins: getAdminMenu(), getAdminRoutes(), handleAdminRoute()
- Content plugins: getSidebarContent() (unchanged)

themes/default/base.twig

@@ -31,7 +31,7 @@
     <link rel="me" href="{{ author_git }}">
 
     <!-- Favicon -->
-    <link rel="icon" href="{{ theme_base_url }}/assets/img/favicon.ico" sizes="any">
+    <link rel="icon" type="image/x-icon" href="{{ theme_base_url }}/assets/img/favicon.ico">
     <link rel="icon" type="image/svg+xml" href="{{ theme_base_url }}/assets/img/favicon.svg">
     <link rel="apple-touch-icon" href="{{ theme_base_url }}/assets/img/favicon.svg">
     <link rel="manifest" href="/manifest.json">