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/partial/PartialHost.vue

@@ -12,7 +12,8 @@ import { renderPartialNodes } from './partialNodes'
 // The first load shows a skeleton; a reload (reloadKey change) keeps the
 // current nodes visible and only marks the host busy. Zero nodes render
 // nothing; a failure shows the extension failure box. No live region: the
-// toast of the action that caused a reload is the announcement.
+// toast of the action that caused a reload is the announcement. A reload
+// that fails keeps nothing: the failure box replaces the content.
 const props = withDefaults(
   defineProps<{
     source: ControllerParams
@@ -22,8 +23,10 @@ const props = withDefaults(
     variant: 'header' | 'field'
     /** Bumped by the parent to refetch. */
     reloadKey?: number
+    /** A status hint (preview screen): the first load shows one 68px block. */
+    hint?: boolean
   }>(),
-  { recordId: null, reloadKey: 0 },
+  { recordId: null, reloadKey: 0, hint: false },
 )
 
 const nodes = ref<PartialNode[] | null>(null)
@@ -66,7 +69,7 @@ void load()
   <div v-else-if="nodes === null" data-partial-loading aria-busy="true">
     <div
       data-partial-skeleton
-      :class="variant === 'header' ? 'h-[80px] w-full rounded-card' : 'h-[44px] w-full rounded-control'"
+      :class="hint ? 'h-[68px] w-full rounded-inner' : variant === 'header' ? 'h-[80px] w-full rounded-card' : 'h-[44px] w-full rounded-control'"
       class="bg-skel"
       aria-hidden="true"
     />