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.form-schema.json

@@ -0,0 +1,29 @@
+{
+  "data": {
+    "name": "Person",
+    "modelClass": "Person",
+    "fields": [
+      { "name": "name", "type": "text", "label": "Name", "span": "left" },
+      { "name": "email", "type": "text", "label": "Email", "span": "right" },
+      { "name": "joined_ip", "type": "text", "label": "Joined from IP address", "context": "preview" }
+    ],
+    "preview": { "headerPartial": "status" },
+    "messages": {
+      "create": { "other": "Person" },
+      "update": { "other": "Edit record" },
+      "saved": { "other": "Saved" },
+      "deleteConfirm": { "other": "Delete this record?" },
+      "deleted": { "other": "Record deleted" },
+      "preview": { "other": "Person details" },
+      "edit": { "other": "Edit person" }
+    },
+    "redirects": {
+      "default": "acme/roster/people",
+      "create": { "redirect": "acme/roster/people/preview/:id", "redirectClose": "acme/roster/people" },
+      "update": { "redirect": "acme/roster/people", "redirectClose": "acme/roster/people/preview/:id" }
+    },
+    "meta": { "locale": "en" },
+    "assets": { "scripts": [], "styles": [] }
+  },
+  "meta": { "locale": "en" }
+}