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

docs/backend/admin-spa.md

@@ -31,6 +31,8 @@ A list whose schema carries declared bulk actions shows a "Bulk actions" menu af
 
 A list row that carries a state (deleted, negative or disabled) shows a text badge for each state after its first cell, together with a text style; the row background is never changed. See [Row state](lists-and-filters.md#row-state).
 
+A form whose schema carries `preview` has a read-only record screen at `<controller>/<id>/preview`, next to the list (`<controller>`), the create form (`<controller>/create`) and the update form (`<controller>/<id>`). It renders the fields whose context allows `preview` as text, the status hint partial above them and, in its footer, the record actions the record response offers before the one edit button. Opening that route for a form without a preview goes to the update form. See [Preview screen](forms.md#preview-screen).
+
 ## Types from OpenAPI
 
 The admin API is described by swag annotations in cabana. `scripts/check-admin-openapi.sh` generates the OpenAPI document (`admin/openapi/admin.json`) from them and the SPA's TypeScript types (`admin/src/api/schema.d.ts`) from the document, so the SPA's API client is checked against the server's shapes at compile time. `--check` fails when either committed file is out of date: