feat(cabana): read-only Plugins screen with installed plugins

- GET /plugins (backend.view_plugins, superusers always pass) lists the
  plugins from party.Installed, or party.Activated IDs for older builds,
  with the built framework version and whether the registry is configured
- Settings -> System -> Plugins entry with the new SettingsEntry.path,
  added only for principals allowed to open it
- PluginsView at /system/plugins with an Installed tab (reka-ui Tabs);
  settings panel, rail, breadcrumbs and page title know the route
- OpenAPI and SPA types regenerated; contract, token-class and
  conformance inventories list the route; en/pl strings; docs

admin/src/app/pageTitle.ts

@@ -18,6 +18,9 @@ export function pageIndicator(route: RouteLocationNormalizedLoaded): string {
   if (route.name === 'account') {
     return t('backend::lang.myaccount.menu_label')
   }
+  if (route.name === 'plugins') {
+    return t('backend::lang.plugins.title')
+  }
   if (route.name === 'settings-form') {
     return settingsEntry(String(route.params.code ?? ''))?.label ?? String(route.params.code ?? '')
   }

admin/src/app/router.ts

@@ -12,6 +12,7 @@ import ReorderView from '../views/ReorderView.vue'
 import SettingsFormView from '../views/SettingsFormView.vue'
 import SettingsIndexView from '../views/SettingsIndexView.vue'
 import AccountView from '../views/AccountView.vue'
+import PluginsView from '../views/PluginsView.vue'
 import NotFoundView from '../views/NotFoundView.vue'
 
 declare module 'vue-router' {
@@ -71,6 +72,8 @@ export function createAdminRouter(history: RouterHistory = createWebHistory(runt
       { path: '/settings/:code', name: 'settings-form', component: SettingsFormView, meta: { shell: true } },
       // My account: the signed-in admin's own profile and password.
       { path: '/account', name: 'account', component: AccountView, meta: { shell: true } },
+      // The read-only Plugins screen (Settings → System, backend.view_plugins).
+      { path: '/system/plugins', name: 'plugins', component: PluginsView, meta: { shell: true } },
       { path: '/:vendor/:plugin/:controller', name: 'list', component: ListView, meta: { shell: true } },
       { path: '/:vendor/:plugin/:controller/create', name: 'create', component: FormView, meta: { shell: true } },
       // The tree reorder page of a list with config_reorder.yaml (D-09).