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/views/pages/error.twig

@@ -0,0 +1,14 @@
+{% extends "layouts/admin.twig" %}
+
+{% block title %}{{ error_title|default('Fout') }} - CodePress Admin{% endblock %}
+
+{% block content %}
+<div class="text-center py-5">
+    <h1 class="display-1 text-muted">{{ error_code|default('404') }}</h1>
+    <h2 class="mb-3">{{ error_title|default('Pagina niet gevonden') }}</h2>
+    <p class="text-muted mb-4">{{ error_message|default('De gevraagde pagina kon niet worden gevonden.') }}</p>
+    <a href="/admin/dashboard" class="btn btn-primary">
+        <i class="bi bi-house"></i> Naar dashboard
+    </a>
+</div>
+{% endblock %}