System plugin support: admin menu items, admin routes, API integration

- PluginManager: getAdminMenuItems(), handleAdminRoute(), getPluginType()
- admin.php: load PluginManager, pass plugin_admin_menu to Twig
- admin.php: route to plugin admin pages via handleAdminRoute()
- admin.twig: show 'Plugins' sidebar section for system plugins
- plugins-new.twig: choose content or system plugin type
- handlePluginsNew: generate proper template based on type (content/system)
- plugin-admin.twig: renders plugin output in admin layout
- GeoIPInfo: example system plugin (admin page with GeoIP info)
- System plugins: getAdminMenu(), getAdminRoutes(), handleAdminRoute()
- Content plugins: getSidebarContent() (unchanged)

admin/theme/default/views/pages/plugins-new.twig

@@ -13,6 +13,21 @@
             <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 class="mb-3">
+            <label class="form-label">Plugin type</label>
+            <div class="form-check">
+                <input class="form-check-input" type="radio" name="type" id="type-content" value="content" checked>
+                <label class="form-check-label" for="type-content">
+                    <strong>Content plugin</strong> — Verschijnt in de sidebar op content pagina's
+                </label>
+            </div>
+            <div class="form-check">
+                <input class="form-check-input" type="radio" name="type" id="type-system" value="system">
+                <label class="form-check-label" for="type-system">
+                    <strong>Systeem plugin</strong> — Voegt functionaliteit toe aan het CMS (admin menu, API)
+                </label>
+            </div>
+        </div>
     </div>
     <div class="card-footer bg-white">
         <button type="submit" class="btn btn-primary">
@@ -21,4 +36,4 @@
         <a href="/admin/plugins" class="btn btn-outline-secondary">Annuleren</a>
     </div>
 </form>
-{% endblock %}
+{% endblock %}