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/content-new.twig

@@ -0,0 +1,42 @@
+{% extends "layouts/admin.twig" %}
+
+{% block title %}Nieuwe pagina - CodePress Admin{% endblock %}
+
+{% block content %}
+<h2 class="mb-4"><i class="bi bi-plus-lg"></i> Nieuwe pagina</h2>
+
+<div class="card shadow-sm">
+    <div class="card-body">
+        <form method="POST" action="/admin/content-new?dir={{ dir|url_encode }}" id="editor-form" data-new-page>
+            <input type="hidden" name="csrf_token" value="{{ csrf_token }}">
+            <div class="mb-3">
+                <label for="filename" class="form-label">Bestandsnaam</label>
+                <input type="text" class="form-control" id="filename" name="filename" required autofocus>
+                <small class="form-text text-muted">Alleen letters, cijfers, punten, underscores en streepjes.</small>
+            </div>
+            <div class="mb-3">
+                <label for="extension" class="form-label">Bestandstype</label>
+                <select class="form-select" id="extension" name="extension">
+                    {% for ext, label in availableExtensions %}
+                    <option value="{{ ext }}">{{ label }}</option>
+                    {% endfor %}
+                </select>
+            </div>
+            <div class="mb-3">
+                <label for="layout" class="form-label">Sjabloon / Layout <small class="text-muted">({{ activeThemeName|default('default') }} theme)</small></label>
+                <select class="form-select" id="layout" name="layout">
+                    {% for key, layoutFile in themeLayouts %}
+                    <option value="{{ key }}" {{ key == themeDefaultLayout ? 'selected' : '' }}>{{ key|replace({'_': ' '})|capitalize }}{% if key == themeDefaultLayout %} (standaard){% endif %}</option>
+                    {% endfor %}
+                </select>
+            </div>
+            <div class="d-flex gap-2">
+                <button type="submit" class="btn btn-primary">
+                    <i class="bi bi-check-lg"></i> Aanmaken
+                </button>
+                <a href="/admin/content?dir={{ dir|url_encode }}" class="btn btn-outline-secondary">Annuleren</a>
+            </div>
+        </form>
+    </div>
+</div>
+{% endblock %}