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;

src/data/schemas/user/user.ts

@@ -22,8 +22,8 @@ export const UserSchema = z.object({
   personalityTraits: PersonalityTraitsSchema.default(PERSONALITY_TRAITS_DEFAULTS),
   preferredLanguage: stringOrEmpty,
   createdAt: stringOrEmpty,
-  // 后端新游客返回 null（"还没发过消息"），保持 nullable（User 类声明 `string | null`）
+  // 后端新游客返回 null（"还没发过消息"），前端统一收敛为空字符串。
-  lastMessageAt: z.string().nullable().default(null),
+  lastMessageAt: stringOrEmpty,
   avatarUrl: stringOrEmpty,
   loginProvider: stringOrEmpty,
   isGuest: booleanOrFalse,