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.md

@@ -0,0 +1,14 @@
+# Theme Developer
+
+The theme developer guide contains the following topics:
+
+- **Theme structure** - Folder structure of a theme
+- **theme.json** - Theme configuration
+- **Twig templates** - Twig syntax, variables and blocks
+- **SCSS styling** - Compiling and writing SCSS
+- **Layouts** - Defining and using layouts
+- **New theme** - Creating a theme via admin or manually
+
+See also the **Content API** guide under Content Manager for all available Twig variables and CMSAPI methods.
+
+Select a topic from the navigation on the left.