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/guide.twig

@@ -0,0 +1,52 @@
+{% extends "layouts/admin.twig" %}
+
+{% block title %}{{ guide_page ? 'Handleiding - ' : '' }}Handleiding{% endblock %}
+
+{% block content %}
+<div class="row">
+    {% if guide_nav %}
+    <aside class="col-md-3 mb-3">
+        <div class="card shadow-sm">
+            <div class="card-header">
+                <h5 class="mb-0"><i class="bi bi-list-ul"></i> Navigatie</h5>
+            </div>
+            <div class="card-body">
+                {{ guide_nav|raw }}
+            </div>
+        </div>
+    </aside>
+    <div class="col-md-9">
+    {% else %}
+    <div class="col-12">
+    {% endif %}
+        <nav aria-label="breadcrumb" class="mb-4">
+            <ol class="breadcrumb">
+                <li class="breadcrumb-item {{ not guide_page ? 'active' : '' }}">
+                    {% if guide_page %}
+                        <a href="/admin/guide{% if guide_lang %}?lang={{ guide_lang }}{% endif %}">Handleiding</a>
+                    {% else %}
+                        Handleidingen
+                    {% endif %}
+                </li>
+                {% if guide_breadcrumbs %}
+                    {% for crumb in guide_breadcrumbs %}
+                        {% if loop.last %}
+                            <li class="breadcrumb-item active">{{ crumb.title }}</li>
+                        {% else %}
+                            <li class="breadcrumb-item">
+                                <a href="/admin/guide?lang={{ guide_lang }}&page={{ crumb.url }}">{{ crumb.title }}</a>
+                            </li>
+                        {% endif %}
+                    {% endfor %}
+                {% endif %}
+            </ol>
+        </nav>
+
+        <div class="guide-content card shadow-sm">
+            <div class="card-body">
+                {{ content|raw }}
+            </div>
+        </div>
+    </div>
+</div>
+{% endblock %}