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

modules/cabana/admin_openapi.go

@@ -479,6 +479,19 @@ func AdminFieldTypes() {}
 // @Router /settings [get]
 func AdminSettingsList() {}
 
+// AdminPluginsInstalled documents GET /plugins.
+//
+// @Summary Installed plugins
+// @Description The plugins the application was built with, in summer.yaml order, with the name, description, version and source summer build baked in (only the ID for an application built before plugin metadata existed), the framework version the application was built with and whether the plugin registry is configured (registry.enabled false and registry.status disabled when plugins.registry_url is empty). Requires backend.view_plugins (superusers always pass).
+// @Tags admin
+// @Produce json
+// @Security BackendBearer
+// @Success 200 {object} Envelope[PluginsOverview]
+// @Failure 401 {object} ErrorEnvelope
+// @Failure 403 {object} ErrorEnvelope
+// @Router /plugins [get]
+func AdminPluginsInstalled() {}
+
 // AdminSettingsSchema documents GET /settings/{code}/schema.
 //
 // @Summary Admin settings schema