fix(user): make lastMessageAt nullable to handle new backend response

src/data/schemas/user/user.ts

@@ -17,7 +17,8 @@ export const UserSchema = z.object({
   personalityTraits: PersonalityTraitsSchema.default(PERSONALITY_TRAITS_DEFAULTS),
   preferredLanguage: z.string().default(""),
   createdAt: z.string().default(""),
-  lastMessageAt: z.string().default(""),
+  // 后端**新**游客返回 null（"还没发过消息"），`.default("")` **不**兜 null —— 需 nullable
+  lastMessageAt: z.string().nullable().default(null),
   avatarUrl: z.string().default(""),
   loginProvider: z.string().default("email"),
   isGuest: z.boolean().default(false),