feat(16.2-01): host plugin field type elements in the admin SPA

- loadFieldTypes fetches GET /field-types after sign-in, loads each script
  once and adds the styles as global links activateStyles never disables
- PluginField creates the type's element, sets the value property and the
  contract attributes, takes summer-change as the form value and never
  writes back a value the element just emitted
- rendererForField and isRegisteredField route any field with an element
  to PluginField in forms, previews and the save payload
- rebuilt modules/boardwalk/dist

admin/src/api/types.ts

@@ -54,6 +54,8 @@ export type AdminActionRequest = Schemas['cabana.AdminActionRequest']
 export type AdminActionResult = Schemas['cabana.AdminActionResult']
 /** A controller's plugin script and stylesheet URLs under {base}/assets/. */
 export type ControllerAssets = Schemas['cabana.ControllerAssets']
+/** A plugin field type: alias, element tag, translatable flag and its asset URLs. */
+export type FieldTypeAssets = Schemas['cabana.FieldTypeAssets']
 /** A registered toolbar action the admin may run, with its localized label. */
 export type ToolbarAction = Schemas['cabana.ToolbarAction']
 /** One node of a rendered partial: an allowlisted element or a text node. */