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-new.twig

@@ -0,0 +1,24 @@
+{% extends "layouts/admin.twig" %}
+
+{% block title %}Nieuw thema - CodePress Admin{% endblock %}
+
+{% block content %}
+<h2 class="mb-4"><i class="bi bi-palette"></i> Nieuw thema aanmaken</h2>
+
+<form method="post" class="card shadow-sm">
+    <input type="hidden" name="csrf_token" value="{{ csrf_token }}">
+    <div class="card-body">
+        <div class="mb-3">
+            <label for="name" class="form-label">Thema naam</label>
+            <input type="text" class="form-control" id="name" name="name" required autofocus>
+            <small class="form-text text-muted">Alleen letters, cijfers, underscores en streepjes.</small>
+        </div>
+    </div>
+    <div class="card-footer bg-white">
+        <button type="submit" class="btn btn-primary">
+            <i class="bi bi-check-lg"></i> Aanmaken
+        </button>
+        <a href="/admin/theme" class="btn btn-outline-secondary">Annuleren</a>
+    </div>
+</form>
+{% endblock %}