v2.5.1: Admin theme refactor, Navigation plugin, user roles, guide restructure

- Reorganize admin into admin/theme/default/ (views + assets)
- Rename GuideNav to Navigation plugin (essential, protected)
- Plugin assets support (SCSS/CSS) loaded after theme CSS
- User roles: Admin, Content Manager, BI Manager, Site Admin
- Role-based access control (RBAC) for admin routes and sidebar
- Guide restructure: sub-topics in separate folders with sidebar nav
- Dynamic breadcrumb for homepage and subdirectories
- Fix theme path traversal (../../ -> ../) in admin.php
- Fix CodeMirror mode load order (xml -> css -> js -> htmlmixed -> php)
- Fix editor-toolbar.js null checks for plugin edit pages
- Layout select from theme.json with live frontmatter update
- Footer sticky at bottom of viewport (min-height: 100vh)
- Breadcrumb color fix (var(--nav-font) -> var(--header-bg))
- Remove language switcher from guide pages
- Update README.md and README.en.md
- Bump version to 2.5.1

cms/core/plugin/PluginManager.php

@@ -174,4 +174,23 @@ class PluginManager
         
         return $sidebarContent;
     }
+
+    /**
+     * Get CSS URLs from all enabled plugins that provide getCssUrl().
+     *
+     * @return array List of CSS URLs
+     */
+    public function getPluginCssUrls(): array
+    {
+        $urls = [];
+        foreach ($this->plugins as $pluginName => $plugin) {
+            if (method_exists($plugin, 'getCssUrl')) {
+                $url = $plugin->getCssUrl();
+                if (!empty($url)) {
+                    $urls[] = $url;
+                }
+            }
+        }
+        return $urls;
+    }
 }