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/update.twig

@@ -0,0 +1,35 @@
+{% extends "layouts/admin.twig" %}
+
+{% block title %}Update - CodePress Admin{% endblock %}
+
+{% block content %}
+<h2 class="mb-4"><i class="bi bi-cloud-arrow-down"></i> Systeem Update</h2>
+
+{% if isGitWritable == false %}
+    <div class="alert alert-warning mb-4">
+        <i class="bi bi-exclamation-triangle-fill me-1"></i>
+        De .git map is niet beschrijfbaar. Automatische updates zijn niet mogelijk.
+    </div>
+{% endif %}
+
+<div class="card shadow-sm mb-4">
+    <div class="card-header">Huidige versie</div>
+    <div class="card-body">
+        <p class="mb-0">CodePress versie: <strong>{{ version }}</strong></p>
+    </div>
+</div>
+
+<div class="card shadow-sm">
+    <div class="card-header">Update opties</div>
+    <div class="card-body">
+        <div class="d-grid gap-2">
+            <button class="btn btn-primary" {{ isGitWritable ? '' : 'disabled' }}>
+                <i class="bi bi-cloud-arrow-down"></i> Controleer op updates
+            </button>
+            <button class="btn btn-outline-secondary" {{ isGitWritable ? '' : 'disabled' }}>
+                <i class="bi bi-arrow-repeat"></i> Update uitvoeren
+            </button>
+        </div>
+    </div>
+</div>
+{% endblock %}