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/config/admin.json.example

@@ -1,15 +0,0 @@
-{
-    "users": [
-        {
-            "username": "admin",
-            "password_hash": "$2y$12$nUpoaCNZZFL8kOTHNC85q.dTy0hWRmPoF3dAn4GcvSXERMioYr5b6",
-            "role": "admin",
-            "created": "2025-01-01"
-        }
-    ],
-    "security": {
-        "session_timeout": 1800,
-        "max_login_attempts": 5,
-        "lockout_duration": 900
-    }
-}