feat: LicenseGuard 中间件 + Flutter license model/repo (21D+21E)

21D — 后端:
- Claims 新增 LicenseExpiresAt (*int64 unix 秒)，写入 JWT 避免每次查库
- middleware/license_guard.go: CalcLicensePhase / LicenseGuard / GetLicensePhase
  - grace(0-7d): 允许通行
  - readonly(7-15d): 拦截非 GET 写操作 → 403
  - locked(15d+): 全部拦截 → 403
- auth.go: issueTokens 在 JWT 中嵌入 license expires_at；Login 检查 locked 拒绝登录
- router: license/* 路由豁免 LicenseGuard（锁定时仍可激活/查状态）

21E — Flutter 前端:
- models/license.dart: 新 LicenseInfo，含 phase/maxDevices，去掉旧 activatedAt
- core/device/device_id.dart: 持久化 UUID-v4 作为设备 ID（SharedPreferences）
- repositories/license_repository.dart: getInfo/activate/deactivate，激活时携带设备信息
- providers/license_provider.dart: 改接 LicenseRepository
- settings_screen.dart: activatedAt 改为显示 maxDevices

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/lib/core/device/device_id.dart

@@ -0,0 +1,53 @@
+import 'dart:math';
+import 'package:flutter/foundation.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+const _kDeviceIdKey = 'device_id';
+
+/// Returns a stable device identifier persisted across app launches.
+/// On first call a random UUID-v4 is generated and stored in SharedPreferences.
+/// Follows the rule: check kIsWeb before dart:io.
+class DeviceId {
+  static String? _cached;
+
+  static Future<String> get() async {
+    if (_cached != null) return _cached!;
+    final prefs = await SharedPreferences.getInstance();
+    var id = prefs.getString(_kDeviceIdKey);
+    if (id == null || id.isEmpty) {
+      id = _generateUuid();
+      await prefs.setString(_kDeviceIdKey, id);
+    }
+    return _cached = id;
+  }
+
+  /// Best-effort platform name for display purposes.
+  static String get platformName {
+    if (kIsWeb) return 'web';
+    // defaultTargetPlatform is safe on all platforms (no dart:io needed)
+    switch (defaultTargetPlatform) {
+      case TargetPlatform.windows:
+        return 'windows';
+      case TargetPlatform.macOS:
+        return 'macos';
+      case TargetPlatform.android:
+        return 'android';
+      case TargetPlatform.iOS:
+        return 'ios';
+      case TargetPlatform.linux:
+        return 'linux';
+      default:
+        return 'unknown';
+    }
+  }
+
+  static String _generateUuid() {
+    final rng = Random.secure();
+    final bytes = List<int>.generate(16, (_) => rng.nextInt(256));
+    bytes[6] = (bytes[6] & 0x0f) | 0x40; // version 4
+    bytes[8] = (bytes[8] & 0x3f) | 0x80; // variant
+    final hex = bytes.map((b) => b.toRadixString(16).padLeft(2, '0')).join();
+    return '${hex.substring(0, 8)}-${hex.substring(8, 12)}-'
+        '${hex.substring(12, 16)}-${hex.substring(16, 20)}-${hex.substring(20)}';
+  }
+}

client/lib/models/license.dart

@@ -0,0 +1,56 @@
+class LicenseInfo {
+  final int id;
+  final String type; // trial | monthly | annual | lifetime
+  final bool isActive;
+  final int maxDevices;
+  final DateTime? expiresAt;
+  final String phase; // normal | grace | readonly | locked
+
+  const LicenseInfo({
+    required this.id,
+    required this.type,
+    required this.isActive,
+    required this.maxDevices,
+    required this.phase,
+    this.expiresAt,
+  });
+
+  factory LicenseInfo.fromJson(Map<String, dynamic> json) {
+    return LicenseInfo(
+      id: (json['id'] as num?)?.toInt() ?? 0,
+      type: json['type'] as String? ?? 'trial',
+      isActive: json['is_active'] as bool? ?? false,
+      maxDevices: (json['max_devices'] as num?)?.toInt() ?? 3,
+      expiresAt: json['expires_at'] != null
+          ? DateTime.tryParse(json['expires_at'] as String)
+          : null,
+      phase: json['phase'] as String? ?? 'normal',
+    );
+  }
+
+  String get typeLabel {
+    switch (type) {
+      case 'monthly':
+        return '月度授权';
+      case 'annual':
+        return '年度授权';
+      case 'lifetime':
+        return '永久授权';
+      default:
+        return '试用版';
+    }
+  }
+
+  bool get isExpired =>
+      expiresAt != null && DateTime.now().isAfter(expiresAt!);
+
+  int? get daysRemaining {
+    if (expiresAt == null) return null;
+    final diff = expiresAt!.difference(DateTime.now()).inDays;
+    return diff < 0 ? 0 : diff;
+  }
+
+  bool get isReadOnlyPhase => phase == 'readonly' || phase == 'locked';
+  bool get isLockedPhase => phase == 'locked';
+  bool get needsAttention => phase == 'grace' || phase == 'readonly' || phase == 'locked';
+}

client/lib/providers/license_provider.dart

@@ -1,53 +1,14 @@
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import '../core/api/api_client.dart';
 import '../core/auth/auth_state.dart';
+import '../models/license.dart';
+import '../repositories/license_repository.dart';
 
-class LicenseInfo {
-  final String type;       // trial / monthly / annual / lifetime
-  final bool isActive;
-  final DateTime? expiresAt;
-  final DateTime? activatedAt;
+export '../models/license.dart';
 
-  const LicenseInfo({
-    required this.type,
-    required this.isActive,
-    this.expiresAt,
-    this.activatedAt,
-  });
-
-  factory LicenseInfo.fromJson(Map<String, dynamic> json) {
-    return LicenseInfo(
-      type: json['type'] as String? ?? 'trial',
-      isActive: json['is_active'] as bool? ?? false,
-      expiresAt: json['expires_at'] != null
-          ? DateTime.tryParse(json['expires_at'] as String)
-          : null,
-      activatedAt: json['activated_at'] != null
-          ? DateTime.tryParse(json['activated_at'] as String)
-          : null,
-    );
-  }
-
-  String get typeLabel {
-    switch (type) {
-      case 'monthly': return '月度授权';
-      case 'annual':  return '年度授权';
-      case 'lifetime': return '永久授权';
-      default:        return '试用版';
-    }
-  }
-
-  /// 是否已过期
-  bool get isExpired =>
-      expiresAt != null && DateTime.now().isAfter(expiresAt!);
-
-  /// 距到期剩余天数（null = 永久）
-  int? get daysRemaining {
-    if (expiresAt == null) return null;
-    final diff = expiresAt!.difference(DateTime.now()).inDays;
-    return diff < 0 ? 0 : diff;
-  }
-}
+final licenseRepositoryProvider = Provider<LicenseRepository>(
+  (ref) => LicenseRepository(ref.read(apiClientProvider)),
+);
 
 final licenseProvider =
     AsyncNotifierProvider<LicenseNotifier, LicenseInfo?>(LicenseNotifier.new);
@@ -61,11 +22,7 @@ class LicenseNotifier extends AsyncNotifier<LicenseInfo?> {
 
   Future<LicenseInfo?> _fetch() async {
     try {
-      final client = ref.read(apiClientProvider);
-      final resp = await client.get('/license/info');
-      final data = resp.data['data'];
-      if (data == null) return null;
-      return LicenseInfo.fromJson(data as Map<String, dynamic>);
+      return await ref.read(licenseRepositoryProvider).getInfo();
     } catch (_) {
       return null;
     }

client/lib/repositories/license_repository.dart

@@ -0,0 +1,55 @@
+import 'package:dio/dio.dart';
+import '../core/api/api_client.dart';
+import '../core/device/device_id.dart';
+import '../core/exceptions.dart';
+import '../models/license.dart';
+
+class LicenseRepository {
+  final ApiClient _client;
+  const LicenseRepository(this._client);
+
+  Future<LicenseInfo?> getInfo() async {
+    try {
+      final resp = await _client.get('/license/info');
+      final data = resp.data['data'];
+      if (data == null) return null;
+      return LicenseInfo.fromJson(data as Map<String, dynamic>);
+    } on DioException catch (e) {
+      throw AppException(
+        e.response?.data?['error'] as String? ?? '获取授权信息失败',
+        statusCode: e.response?.statusCode,
+      );
+    }
+  }
+
+  /// Activate a license key for this device.
+  Future<void> activate(String licenseKey, {String? deviceName}) async {
+    final deviceId = await DeviceId.get();
+    try {
+      await _client.post('/license/activate', data: {
+        'license_key': licenseKey,
+        'device_id': deviceId,
+        'device_name': deviceName ?? DeviceId.platformName,
+        'platform': DeviceId.platformName,
+      });
+    } on DioException catch (e) {
+      throw AppException(
+        e.response?.data?['error'] as String? ?? '激活失败',
+        statusCode: e.response?.statusCode,
+      );
+    }
+  }
+
+  /// Deactivate (unbind) this device from its license.
+  Future<void> deactivate() async {
+    final deviceId = await DeviceId.get();
+    try {
+      await _client.post('/license/deactivate', data: {'device_id': deviceId});
+    } on DioException catch (e) {
+      throw AppException(
+        e.response?.data?['error'] as String? ?? '解绑失败',
+        statusCode: e.response?.statusCode,
+      );
+    }
+  }
+}

client/lib/screens/settings/settings_screen.dart

@@ -381,10 +381,7 @@ class _SettingsScreenState extends ConsumerState<SettingsScreen> {
                             value: DateFormat('yyyy-MM-dd').format(lic.expiresAt!))
                       else
                         const _ParamRow(label: '到期时间', value: '永久有效'),
-                      if (lic.activatedAt != null)
-                        _ParamRow(
-                            label: '激活时间',
-                            value: DateFormat('yyyy-MM-dd').format(lic.activatedAt!)),
+                      _ParamRow(label: '设备上限', value: '${lic.maxDevices} 台'),
                       const SizedBox(height: 12),
                       OutlinedButton.icon(
                         onPressed: _showRenewLicenseDialog,