feat(12.1-02): read-only preview screen with a status hint and record actions

- config_form.yaml preview block (optional headerPartial), reported in the form schema as preview
- fields with context: preview show only on the preview screen and are never written
- form messages preview and edit; recordActions without a preview block stops boot
- SPA route {id}/preview, PreviewView and PreviewField, record actions in the footer
- mapWinterUrl maps preview/:id; the update form returns to the preview
- summer-callout partial style classes for status hints
- README, docs, OpenAPI document, TS types and the embedded build updated

modules/cabana/openapi_conformance_test.go

@@ -916,6 +916,7 @@ update:
     redirect: acme/conform/gadgets
     redirectClose: acme/conform/gadgets
 recordActions: [ping]
+preview: {}
 `),
 		"controllers/gadgets/config_relation.yaml": file(`members:
     label: Members