fix(client): 补提交 stats-overhaul 漏掉的 account_api/kernel_process WIP

同 device_usage:account_api.dart(deviceUsage/usage 方法)、kernel_process.dart
及对应测试一直未提交,本地有→analyze/test 过,但不在 git→Windows 构建报
「deviceUsage isn't defined」。补齐使提交树自洽可构建。
孤儿组件 device_stat_row/metric_card 已无人引用,不提交。

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

client/lib/services/account_api.dart

@@ -3,6 +3,7 @@
 // 覆盖:/v1/me、/v1/plans、/v1/me/devices(列表+删除)、/v1/usage、
 // /v1/redeem、/v1/ads/unlock。错误统一 AuthApiException(由 ApiClient 抛)。
 import '../models/device.dart';
+import '../models/device_usage.dart';
 import '../models/me.dart';
 import '../models/plan.dart';
 import '../models/usage_point.dart';
@@ -65,6 +66,13 @@ class AccountApi {
     return raw.map((e) => UsagePoint.fromJson(e as Map<String, dynamic>)).toList();
   }
 
+  /// GET /v1/usage/devices?days=N — 最近 N 天按设备用量(默认 7,范围 [1,90])。
+  Future<List<DeviceUsage>> deviceUsage({int days = 7}) async {
+    final body = await _c.getJson('/v1/usage/devices?days=$days');
+    final raw = (body['devices'] as List<dynamic>?) ?? const [];
+    return raw.map((e) => DeviceUsage.fromJson(e as Map<String, dynamic>)).toList();
+  }
+
   /// POST /v1/redeem — 兑换码。
   Future<RedeemResult> redeem(String code) async =>
       RedeemResult.fromJson(await _c.postJson('/v1/redeem', {'code': code}));