Show persistent timestamps for operation and teacher messages

shared/message-timestamp.ts

@@ -0,0 +1,7 @@
+/** Read only real, valid epoch milliseconds / ISO dates; never invent a history time. */
+export function messageTimestamp(value: unknown): number | undefined {
+  const timestamp = typeof value === 'number' ? value
+    : typeof value === 'string' && /^\d{4}-\d{2}-\d{2}T/.test(value) ? Date.parse(value) : NaN;
+  return Number.isFinite(timestamp) && timestamp >= 0 && !Number.isNaN(new Date(timestamp).getTime())
+    ? timestamp : undefined;
+}