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

admin/src/components/form/formState.ts

@@ -3,7 +3,8 @@
 import type { AdminRecord, ErrorBody, FormField } from '../../api/types'
 import { isRegistered } from './registry'
 
-export type FormMode = 'create' | 'update'
+/** The screen a field is filtered for: the two form modes and the read-only preview (D-11). */
+export type FormMode = 'create' | 'update' | 'preview'
 
 /** One form tab: the YAML tab label, or the default tab for untabbed fields. */
 export interface TabItem {