chore(auth): add debug logging to guest login flow

Add console.log instrumentation across the guest login pipeline (auth-machine, authRepository, authApi) to trace request lifecycle and surface Zod parsing failures with detailed issue info. Useful for diagnosing lastMessageAt schema mismatches.

src/data/services/api/auth_api.ts

@@ -4,6 +4,8 @@
  * 认证相关接口
  * 原始 Dart: lib/data/services/api/auth_api_client.dart
  */
+import { z } from "zod";
+
 import { httpClient } from "./http_client";
 import { ApiPath } from "./api_path";
 import { ApiEnvelope, unwrap } from "./response_helper";
@@ -117,7 +119,29 @@ export class AuthApi {
       method: "POST",
       body: body.toJson(),
     });
-    return GuestLoginResponse.fromJson(unwrap(env) as Record<string, unknown>);
+    console.log("[AuthApi.guestLogin] raw envelope from backend", env);
+    const unwrapped = unwrap(env) as Record<string, unknown>;
+    const userObj = unwrapped.user as { lastMessageAt?: unknown } | undefined;
+    console.log("[AuthApi.guestLogin] unwrapped data (pre-parse)", {
+      hasUser: "user" in unwrapped,
+      userKeys: userObj && typeof userObj === "object" ? Object.keys(userObj) : null,
+      lastMessageAt: userObj?.lastMessageAt,
+      lastMessageAtType: typeof userObj?.lastMessageAt,
+    });
+    try {
+      return GuestLoginResponse.fromJson(unwrapped);
+    } catch (e) {
+      if (e instanceof z.ZodError) {
+        console.error("[AuthApi.guestLogin] ZodError parsing response", {
+          issueCount: e.issues.length,
+          firstIssue: e.issues[0],
+          allPaths: e.issues.map((i) => i.path),
+        });
+      } else {
+        console.error("[AuthApi.guestLogin] unexpected error", e);
+      }
+      throw e;
+    }
   }
 
   /**