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

guide/en/theme-developer/theme-json.md

@@ -0,0 +1,21 @@
+# theme.json
+
+```json
+{
+  "title": "My Theme",
+  "default_layout": "full_content",
+  "header_color": "#0a369d",
+  "layouts": {
+    "full_content": "full_content.twig",
+    "left_sidebar": "left_sidebar.twig",
+    "right_sidebar": "right_sidebar.twig"
+  }
+}
+```
+
+## Fields
+
+- `title` - Display name in admin
+- `default_layout` - Default layout for new pages
+- `header_color` - Admin sidebar color
+- `layouts` - Mapping of layout names to .twig files