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)}';
+  }
+}