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/tests/fixtures/roster.record.json

@@ -1,5 +1,5 @@
 {
-  "data": { "id": 1, "name": "Ada Lovelace", "email": "ada@example.test" },
+  "data": { "id": 1, "name": "Ada Lovelace", "email": "ada@example.test", "joined_ip": "203.0.113.7" },
   "meta": {
     "labels": {},
     "actions": [