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/test/golden/tablet_pages_golden_test.dart

@@ -11,6 +11,7 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:pangolin_vpn/bridge/vpn_bridge_mock.dart';
 import 'package:pangolin_vpn/bridge/vpn_bridge_provider.dart';
 import 'package:pangolin_vpn/l10n/app_text.dart';
+import 'package:pangolin_vpn/models/device_usage.dart';
 import 'package:pangolin_vpn/models/me.dart';
 import 'package:pangolin_vpn/models/node.dart';
 import 'package:pangolin_vpn/models/usage_point.dart';
@@ -73,6 +74,13 @@ final List<UsagePoint> _demoUsage = List.generate(30, (i) {
   );
 });
 
+// 分设备用量演示数据(统计页「设备明细」section)：三台设备,流量递减以验证占比条。
+const List<DeviceUsage> _demoDeviceUsage = [
+  DeviceUsage(uuid: 'd1', name: 'iPhone 15', platform: 'ios', bytesUp: 1500 * 1024 * 1024, bytesDown: 9800 * 1024 * 1024, minutesUsed: 312),
+  DeviceUsage(uuid: 'd2', name: 'MacBook Pro', platform: 'macos', bytesUp: 800 * 1024 * 1024, bytesDown: 5200 * 1024 * 1024, minutesUsed: 188),
+  DeviceUsage(uuid: 'd3', name: 'Pixel 8', platform: 'android', bytesUp: 200 * 1024 * 1024, bytesDown: 1400 * 1024 * 1024, minutesUsed: 64),
+];
+
 class _FakeNodesNotifier extends NodesNotifier {
   @override
   Future<List<Node>> build() async => _demoNodes;
@@ -108,6 +116,7 @@ Future<void> _shoot(
         nodesProvider.overrideWith(_FakeNodesNotifier.new),
         meProvider.overrideWith(_FakeMeNotifier.new),
         usageProvider(30).overrideWith((ref) async => _demoUsage),
+        deviceUsageProvider(30).overrideWith((ref) async => _demoDeviceUsage),
       ],
       child: MaterialApp(
         debugShowCheckedModeBanner: false,