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

admin/theme/default/assets/img/favicon.svg

@@ -0,0 +1,11 @@
+<svg width="32" height="32" viewBox="0 0 32 32" xmlns="http://www.w3.org/2000/svg">
+  <!-- < -->
+  <path d="M8 8 L3 16 L8 24" stroke="#ffffff" stroke-width="3" fill="none" stroke-linecap="round" stroke-linejoin="round"/>
+  
+  <!-- / -->
+  <path d="M12 24 L18 8" stroke="#ffffff" stroke-width="3" stroke-linecap="round"/>
+  
+  <!-- .. -->
+  <circle cx="22" cy="20" r="2" fill="#ffffff"/>
+  <circle cx="28" cy="20" r="2" fill="#ffffff"/>
+</svg>