fix(client): 补提交漏掉的 device_usage 模型 + 测试(Windows 构建缺文件)

device_usage.dart 一直是 untracked(stats-overhaul WIP 漏提交),本地有所以
analyze/test 过,但不在 git → bundle → Windows 构建报「找不到 device_usage.dart /
DeviceUsage isn't a type」。account_providers/account_api/golden 测试都引用它,补提交。

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

client/lib/models/device_usage.dart

@@ -0,0 +1,33 @@
+// device_usage.dart — 单设备窗口用量(GET /v1/usage/devices?days=N 的一条)。
+// 统计页「下分设备」明细的数据载体,与账户聚合的 UsagePoint 对应。
+class DeviceUsage {
+  const DeviceUsage({
+    required this.uuid,
+    required this.name,
+    required this.platform,
+    this.bytesUp = 0,
+    this.bytesDown = 0,
+    this.minutesUsed = 0,
+  });
+
+  final String uuid;
+  final String name;
+
+  /// ios | android | windows | macos。
+  final String platform;
+  final int bytesUp;
+  final int bytesDown;
+  final int minutesUsed;
+
+  int get bytesTotal => bytesUp + bytesDown;
+  double get gbTotal => bytesTotal / (1024 * 1024 * 1024);
+
+  factory DeviceUsage.fromJson(Map<String, dynamic> m) => DeviceUsage(
+        uuid: m['uuid'] as String? ?? '',
+        name: m['name'] as String? ?? '',
+        platform: m['platform'] as String? ?? '',
+        bytesUp: (m['bytes_up'] as num?)?.toInt() ?? 0,
+        bytesDown: (m['bytes_down'] as num?)?.toInt() ?? 0,
+        minutesUsed: (m['minutes_used'] as num?)?.toInt() ?? 0,
+      );
+}

client/test/unit/device_usage_test.dart

@@ -0,0 +1,33 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:pangolin_vpn/models/device_usage.dart';
+
+void main() {
+  group('DeviceUsage.fromJson', () {
+    test('parses a full row and derives totals', () {
+      final d = DeviceUsage.fromJson(const {
+        'uuid': 'dev-a',
+        'name': 'My iPhone',
+        'platform': 'ios',
+        'bytes_up': 1500,
+        'bytes_down': 2500,
+        'minutes_used': 8,
+      });
+      expect(d.uuid, 'dev-a');
+      expect(d.name, 'My iPhone');
+      expect(d.platform, 'ios');
+      expect(d.bytesUp, 1500);
+      expect(d.bytesDown, 2500);
+      expect(d.minutesUsed, 8);
+      expect(d.bytesTotal, 4000);
+      expect(d.gbTotal, 4000 / (1024 * 1024 * 1024));
+    });
+
+    test('tolerates missing/absent fields with zero defaults', () {
+      final d = DeviceUsage.fromJson(const {'uuid': 'dev-b', 'name': '', 'platform': ''});
+      expect(d.bytesUp, 0);
+      expect(d.bytesDown, 0);
+      expect(d.minutesUsed, 0);
+      expect(d.bytesTotal, 0);
+    });
+  });
+}