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/styles/main.css

@@ -273,6 +273,39 @@
     color: var(--c-text);
     font-variant-numeric: tabular-nums;
   }
+
+  /* Callout (Phase 12.1, UI-SPEC): a status hint above a preview screen. */
+  .summer-callout {
+    display: flex;
+    flex-direction: column;
+    gap: 4px;
+    margin: 0;
+    padding: 14px 18px;
+    border-radius: 12px;
+    font-size: 14px;
+    line-height: 1.5;
+    overflow-wrap: anywhere;
+  }
+
+  .summer-callout--warning {
+    background: var(--c-sel);
+    color: var(--c-text);
+  }
+
+  .summer-callout--danger {
+    background: var(--c-danger-soft);
+    color: var(--c-danger);
+  }
+
+  .summer-callout__title {
+    margin: 0;
+    font-weight: 600;
+  }
+
+  .summer-callout__text {
+    margin: 0;
+    font-weight: 400;
+  }
 }
 
 @media (prefers-reduced-motion: reduce) {