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

admin/theme/default/assets/css/style.css

@@ -0,0 +1,27 @@
+/* Admin theme styles */
+
+/* Code block styling (for guide pages) */
+pre {
+    background: #f8f9fa;
+    padding: 1rem;
+    border-radius: 6px;
+    overflow-x: auto;
+    border: 1px solid #dee2e6;
+    margin-bottom: 1rem;
+}
+
+pre code {
+    background: none;
+    padding: 0;
+    color: #333;
+    font-size: 0.85rem;
+    line-height: 1.5;
+}
+
+code {
+    background: #e8e8e8;
+    padding: 0.15rem 0.4rem;
+    border-radius: 3px;
+    font-size: 0.9em;
+    color: #d63384;
+}