fix(user): normalize last message timestamp

src/data/dto/user/user.ts

@@ -20,7 +20,7 @@ export class User {
   declare readonly personalityTraits: PersonalityTraits;
   declare readonly preferredLanguage: string;
   declare readonly createdAt: string;
-  declare readonly lastMessageAt: string | null;
+  declare readonly lastMessageAt: string;
   declare readonly avatarUrl: string;
   declare readonly loginProvider: string;
   declare readonly isGuest: boolean;