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

themes/demo/base.twig

@@ -30,16 +30,19 @@
     <link rel="author" href="{{ author_website }}">
     <link rel="me" href="{{ author_git }}">
 
-    <!-- Favicon and PWA -->
-    <link rel="icon" type="image/svg+xml" href="/assets/favicon.svg">
+    <!-- Favicon -->
+    <link rel="icon" href="{{ theme_base_url }}/assets/img/favicon.ico" sizes="any">
+    <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">
     <meta name="theme-color" content="{{ theme_config.header_color|default('#613583') }}">
 
-    <!-- Styles -->
-    <link href="/assets/css/bootstrap.min.css" rel="stylesheet">
-    <link href="/assets/css/bootstrap-icons.css" rel="stylesheet">
-    <link href="/assets/css/style.css" rel="stylesheet">
-    <link href="/assets/css/mobile.css" rel="stylesheet">
+    <!-- Theme CSS -->
+    {% for cssFile in theme_css_files %}
+    <link href="{{ cssFile }}" rel="stylesheet">
+    {% endfor %}
+    
+    <!-- Compiled theme CSS (if SCSS was used) -->
     {% if theme_css_url %}
     <link href="{{ theme_css_url }}" rel="stylesheet">
     {% endif %}
@@ -127,8 +130,12 @@
         {% include 'partials/footer.twig' %}
     </footer>
 
-    <script src="/assets/js/bootstrap.bundle.min.js"></script>
-    <script src="/assets/js/app.js"></script>
+    <!-- Theme JS -->
+    {% for jsFile in theme_js_files %}
+    <script src="{{ jsFile }}"></script>
+    {% endfor %}
+    
+    <!-- Theme-specific JS -->
     {% if theme_js_url %}
     <script src="{{ theme_js_url }}"></script>
     {% endif %}