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/widgets.form-schema.json

@@ -138,6 +138,12 @@
       },
       "deleted": {
         "other": "Widget deleted"
+      },
+      "preview": {
+        "other": "Record preview"
+      },
+      "edit": {
+        "other": "Edit record"
       }
     },
     "meta": {