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/scss-styling.md

@@ -0,0 +1,24 @@
+# SCSS styling
+
+## Compiling theme.scss
+
+1. Go to **Admin** → **Theme**
+2. Click **Compile SCSS** for your theme
+3. CSS is generated in `assets/css/theme.css`
+
+## SCSS example
+
+```scss
+$primary-color: #0a369d;
+$font-stack: Helvetica, Arial, sans-serif;
+
+body {
+    font-family: $font-stack;
+    color: #333;
+}
+
+.header {
+    background: $primary-color;
+    color: #fff;
+}
+```