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/theme.twig

@@ -0,0 +1,66 @@
+{% extends "layouts/admin.twig" %}
+
+{% block title %}Thema's - CodePress Admin{% endblock %}
+
+{% block content %}
+<div class="d-flex justify-content-between align-items-center mb-4">
+    <h2><i class="bi bi-palette"></i> Thema's</h2>
+    <div>
+        <form method="POST" action="/admin/theme" class="d-inline">
+            <input type="hidden" name="csrf_token" value="{{ csrf_token }}">
+            <button type="submit" name="activate_default" class="btn btn-primary btn-sm">
+                <i class="bi bi-check-lg"></i> Activeer Default
+            </button>
+        </form>
+        <a href="/admin/theme-new" class="btn btn-outline-primary btn-sm ms-2">
+            <i class="bi bi-plus-lg"></i> Nieuw thema
+        </a>
+    </div>
+</div>
+
+<div class="row g-4">
+    {% for theme in themes %}
+    <div class="col-md-6 col-lg-4">
+        <div class="card shadow-sm h-100 {{ theme.active ? 'border-primary border-2' : '' }}">
+            <div class="card-header d-flex justify-content-between align-items-center">
+                <span class="fw-bold">{{ theme.title|default(theme.name) }}</span>
+                {% if theme.active %}
+                <span class="badge bg-success">Actief</span>
+                {% endif %}
+            </div>
+            <div class="card-body">
+                <p class="text-muted small">Naam: {{ theme.name }}</p>
+                {% if theme.default_layout %}
+                <p class="text-muted small">Default layout: {{ theme.default_layout }}</p>
+                {% endif %}
+            </div>
+            <div class="card-footer bg-white">
+                {% if not theme.active %}
+                <form method="POST" action="/admin/theme" class="d-inline">
+                    <input type="hidden" name="csrf_token" value="{{ csrf_token }}">
+                    <input type="hidden" name="activate" value="{{ theme.name }}">
+                    <button type="submit" class="btn btn-sm btn-outline-primary">
+                        <i class="bi bi-check-lg"></i> Activeren
+                    </button>
+                </form>
+                {% endif %}
+                <form method="POST" action="/admin/theme" class="d-inline">
+                    <input type="hidden" name="csrf_token" value="{{ csrf_token }}">
+                    <input type="hidden" name="compile_scss" value="1">
+                    <input type="hidden" name="theme" value="{{ theme.name }}">
+                    <button type="submit" class="btn btn-sm btn-outline-success">
+                        <i class="bi bi-palette"></i> SCSS compileren
+                    </button>
+                </form>
+            </div>
+        </div>
+    </div>
+    {% else %}
+    <div class="col-12">
+        <div class="alert alert-info">
+            <i class="bi bi-info-circle"></i> Geen thema's gevonden.
+        </div>
+    </div>
+    {% endfor %}
+</div>
+{% endblock %}