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)

admin/theme/default/views/layouts/admin.twig

@@ -4,6 +4,7 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>{% block title %}CodePress Admin{% endblock %}</title>
+    <link rel="icon" type="image/x-icon" href="/admin/assets/img/favicon.ico">
     <link rel="stylesheet" href="/admin/assets/css/bootstrap.min.css">
     <link rel="stylesheet" href="/admin/assets/css/bootstrap-icons.css">
     <link rel="stylesheet" href="/admin/assets/css/style.css">
@@ -124,6 +125,17 @@
             {% endif %}
             {% endif %}
 
+            {% if plugin_admin_menu is not empty %}
+            <li class="nav-section">Plugins</li>
+            {% for item in plugin_admin_menu %}
+            <li class="nav-item">
+                <a class="nav-link {{ route == item.route ? 'active' : '' }}" href="/admin/{{ item.route }}">
+                    <i class="bi {{ item.icon|default('bi-gear') }}"></i> {{ item.label }}
+                </a>
+            </li>
+            {% endfor %}
+            {% endif %}
+
             {% if has_permission('guide') %}
             <li class="nav-section">Help</li>
             <li class="nav-item">