feat(preview): stamped history for message times

preview/fake-client.ts

@@ -206,6 +206,16 @@ const INFO: Record<string, Partial<ConversationInfo>> = {
   [FORK_MERGE]: { parent: MASTER },
 };
 
+// Ten minutes apart, ending now: enough to show a time on every message.
+const STEP_MS = 600_000;
+function stamped(messages: HistoryMessage[]): HistoryMessage[] {
+  const end = Date.now();
+  return messages.map((message, index) => ({
+    ...message,
+    ts: new Date(end - (messages.length - index) * STEP_MS).toISOString(),
+  }));
+}
+
 const HISTORY: Record<string, HistoryMessage[]> = {
   [MASTER]: [
     { content: "с утра: дневник приехал? что в хендауте", role: "user" },
@@ -387,7 +397,7 @@ export class FakeClient extends ApiClient {
       pattern: /^\/api\/conversations\/([^/]+)$/,
     },
     {
-      handle: ([, id]) => ({ id, messages: HISTORY[id] ?? [] }),
+      handle: ([, id]) => ({ id, messages: stamped(HISTORY[id] ?? []) }),
       method: "GET",
       pattern: /^\/api\/conversations\/([^/]+)\/history$/,
     },