feat(client): P1 真实数据接入地基(#6 6A)

- api_config.dart:API 基址单源(消除各处重复 _kApiUrl)
- api_client.dart:统一鉴权 HTTP 客户端,401→AuthNotifier.refresh→重试一次,
  错误统一 AuthApiException;+ 单测覆盖 401 刷新重试
- 模型 me/device/plan/usage_point(对齐后端 snake_case 契约)
- account_api.dart:/v1/me、/v1/plans、/v1/me/devices(列表+删)、/v1/usage、
  /v1/redeem、/v1/ads/unlock 封装
- auth_provider:注入 AuthApi + refresh();account_providers 装配 ApiClient/AccountApi
- 顺带修复 onboarding 提交遗留的测试 stub(_NullTokenStore 缺 isOnboarded/markOnboarded)

flutter analyze 0 error;114 tests passed。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/lib/models/device.dart

@@ -0,0 +1,28 @@
+// device.dart — 已登录设备(GET /v1/me/devices)。
+class Device {
+  const Device({
+    required this.uuid,
+    required this.name,
+    required this.platform,
+    this.lastSeen,
+  });
+
+  final String uuid;
+  final String name;
+
+  /// 'ios' | 'android' | 'windows' | 'macos'。
+  final String platform;
+
+  /// 最近活跃(UTC);从未上线为 null。
+  final DateTime? lastSeen;
+
+  factory Device.fromJson(Map<String, dynamic> m) {
+    final ls = m['last_seen'] as String?;
+    return Device(
+      uuid: m['uuid'] as String? ?? '',
+      name: m['name'] as String? ?? '',
+      platform: m['platform'] as String? ?? '',
+      lastSeen: (ls != null && ls.isNotEmpty) ? DateTime.tryParse(ls) : null,
+    );
+  }
+}