v2.6.0: Content backup/git versioning, plugin type system, docs update

New features:
- ContentBackup class with ZIP backup/restore and git versioning
- Admin backup & restore page (content-backup.twig) with git init/commit/log/restore
- Plugin type system: system (blue) vs content (green) with visual badges
- PluginAPIInterface + AdminPluginAPI for plugin architecture
- Essential plugin flag (cannot edit/deactivate/delete)

Improvements:
- Consolidated enabled_plugins config (removed plugins.enabled)
- Removed Analytics/Logging toggles from admin config page
- Fixed Dashboard plugin Twig comments rendered as text
- Updated 20 guide files (NL+EN): configuratie, plugins, plugin-development,
  core-classes, theme-json, layouts, scss-styling, admin-beheerder, nieuw-thema, architectuur
- Improved accessibility test script (grep -E, min/max checks)

Cleanup:
- Removed unused classes: ARIAComponents, AccessibilityManager, ContentSecurityPolicy, etc.
- Removed vendor packages: mustache/mustache, php-mqtt/client
- Removed old templates: logs.twig, statistics.twig (now plugins)
- Moved language files to language/ directory

Tests:
- Pentest: 30/30 passed, 0 vulnerabilities
- WCAG 2.1 AA: 25/25 passed, 100% compliance

admin/theme/default/views/pages/content-edit.twig

@@ -1,6 +1,6 @@
 {% extends "layouts/admin.twig" %}
 
-{% block title %}{{ fileName }} - CodePress Admin{% endblock %}
+{% block title %}{{ fileName }} - {{ ta.admin_title|default('CodePress Admin') }}{% endblock %}
 
 {% block content %}
 <h2 class="mb-4"><i class="bi bi-pencil"></i> {{ fileName }}</h2>
@@ -11,16 +11,16 @@
             <input type="hidden" name="csrf_token" value="{{ csrf_token }}">
             <div class="row mb-3">
                 <div class="col-md-4">
-                    <label for="filename" class="form-label">Bestandsnaam</label>
+                    <label for="filename" class="form-label">{{ ta.filename|default('Bestandsnaam') }}</label>
                     <div class="input-group">
                         <input type="text" class="form-control" id="filename" name="filename" value="{{ fileName|replace({'.md': '', '.php': '', '.html': ''}) }}" required>
                         <span class="input-group-text">.{{ fileExt }}</span>
                     </div>
-                    <small class="form-text text-muted">Alleen letters, cijfers, punten, underscores en streepjes.</small>
+                    <small class="form-text text-muted">{{ ta.name_help|default('Alleen letters, cijfers, punten, underscores en streepjes.') }}</small>
                 </div>
                 {% if isEditable %}
                 <div class="col-md-4">
-                    <label for="layout" class="form-label">Sjabloon / Layout <small class="text-muted">({{ activeThemeName|default('default') }} theme)</small></label>
+                    <label for="layout" class="form-label">{{ ta.template_layout|default('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 }}" {{ currentLayout == key ? 'selected' : '' }}>{{ key|replace({'_': ' '})|capitalize }}{% if key == themeDefaultLayout %} (standaard){% endif %}</option>
@@ -29,7 +29,7 @@
                 </div>
                 {% if availablePlugins is not empty %}
                 <div class="col-md-4">
-                    <label class="form-label d-block">Zichtbare plugins</label>
+                    <label class="form-label d-block">{{ ta.visible_plugins|default('Zichtbare plugins') }}</label>
                     <div class="d-flex flex-wrap gap-1">
                         {% for plugin in availablePlugins %}
                         <input type="checkbox" class="btn-check" id="plugin-{{ plugin }}" name="plugins[]" value="{{ plugin }}" autocomplete="off" {{ plugin in selectedPlugins ? 'checked' : '' }}>
@@ -47,15 +47,15 @@
             </div>
             {% endif %}
             <div class="d-flex gap-2 mt-3">
-                <button type="submit" class="btn btn-primary" title="Opslaan (Ctrl+S)">
-                    <i class="bi bi-check-lg"></i> Opslaan
+                <button type="submit" class="btn btn-primary" title="{{ ta.save_ctrl_s|default('Opslaan (Ctrl+S)') }}">
+                    <i class="bi bi-check-lg"></i> {{ ta.save|default('Opslaan') }}
                 </button>
                 {% if isEditable %}
-                <a href="/{{ currentLang }}{% if fileDir %}/{{ fileDir }}{% endif %}/{{ fileName|replace({'.md': '', '.php': '', '.html': ''}) }}" target="_blank" class="btn btn-outline-info" title="Open in nieuw tabblad">
-                    <i class="bi bi-eye"></i> Preview
+                <a href="/{{ currentLang }}{% if fileDir %}/{{ fileDir }}{% endif %}/{{ fileName|replace({'.md': '', '.php': '', '.html': ''}) }}" target="_blank" class="btn btn-outline-info" title="{{ ta.open_new_tab|default('Open in nieuw tabblad') }}">
+                    <i class="bi bi-eye"></i> {{ ta.preview|default('Preview') }}
                 </a>
                 {% endif %}
-                <a href="/admin/content?dir={{ fileDir|url_encode }}" class="btn btn-outline-secondary" id="back-btn">Terug</a>
+                <a href="/admin/content?dir={{ fileDir|url_encode }}" class="btn btn-outline-secondary" id="back-btn">{{ ta.back|default('Terug') }}</a>
             </div>
         </form>
     </div>
@@ -121,4 +121,4 @@ document.addEventListener('DOMContentLoaded', function () {
     window.__onContentChange = markChanged;
 });
 </script>
-{% endblock %}
+{% endblock %}