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

plugins/Navigation/assets/css/navigation.css

@@ -0,0 +1,57 @@
+/* Navigation plugin - compiled from assets/scss/navigation.scss */
+.nav-plugin {
+    padding-left: 0;
+    margin-bottom: 0;
+}
+.nav-plugin-item {
+    margin-bottom: 0.125rem;
+}
+.nav-plugin-link {
+    display: block;
+    padding: 0.3rem 0.5rem;
+    color: #212529;
+    text-decoration: none;
+    border-radius: 0.25rem;
+    border-left: 3px solid transparent;
+    font-size: 0.9rem;
+}
+.nav-plugin-link:hover {
+    background-color: #f8f9fa;
+    text-decoration: none;
+}
+.nav-plugin-link.active {
+    background-color: #e7f1ff;
+    border-left-color: #0a369d;
+    color: #0a369d;
+    font-weight: 600;
+}
+.nav-plugin-icon {
+    font-size: 0.8em;
+    margin-right: 0.25rem;
+    opacity: 0.6;
+}
+.nav-parent {
+    font-weight: 600;
+}
+.nav-parent .nav-plugin-icon {
+    color: #0a369d;
+    opacity: 1;
+}
+.nav-sub {
+    margin-left: 1.1rem;
+    margin-top: 0.2rem;
+    padding-left: 0.6rem;
+    border-left: 1px solid #dee2e6;
+}
+.nav-sub .nav-plugin-link {
+    padding: 0.2rem 0.5rem;
+    font-size: 0.85rem;
+    font-weight: normal;
+    color: #495057;
+}
+.nav-sub .nav-plugin-link.active {
+    color: #0a369d;
+}
+.nav-sub .nav-plugin-icon {
+    font-size: 0.75em;
+}