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/nieuw-thema.md

@@ -0,0 +1,25 @@
+# Creating a new theme
+
+## Via Admin
+
+1. Go to **Theme** → **New theme**
+2. Enter a name (e.g. `my-theme`)
+3. Theme is created with basic structure
+4. Edit `theme.json` and templates
+
+## Manually
+
+```bash
+mkdir themes/my-theme
+mkdir themes/my-theme/partials
+mkdir themes/my-theme/assets/{scss,css,js,fonts,img}
+```
+
+Create basic files:
+
+- `theme.json`
+- `base.twig`
+- `full_content.twig`
+- `partials/header.twig`
+- `partials/footer.twig`
+- `scss/theme.scss`