feat(client): 授权 Tab 激活 UI + 到期横幅/弹窗 (21F+21G)

21F — 激活与设备管理 UI:
- settings_screen: 重构授权 Tab，按 phase 展示色阶状态
  (normal=绿/grace=橙/readonly|locked=红)
- 新增「激活授权码」卡片：文本框 + 激活按钮，调用 LicenseRepository.activate
- 显示设备上限字段 (maxDevices)

21G — gating 横幅 + 到期弹窗:
- app_shell: 引入 licenseProvider，needsAttention 时展示顶部横幅
  (grace=深橙/readonly=深红/locked=红)，横幅含「去激活」快捷入口
- 每 session 首次进入时弹出到期提醒 dialog（postFrameCallback 模式）
  内容随 phase 调整文案，提供「稍后处理」/「立即前往设置」两个按钮

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

client/lib/screens/shell/app_shell.dart

@@ -13,6 +13,7 @@ import '../../providers/connectivity_provider.dart';
 import '../../providers/shop_provider.dart';
 import '../../providers/update_provider.dart';
 import '../../core/update/app_updater.dart';
+import '../../providers/license_provider.dart';
 
 class AppShell extends ConsumerStatefulWidget {
   final Widget child;
@@ -26,6 +27,7 @@ class _AppShellState extends ConsumerState<AppShell> {
   bool _sidebarExpanded = true;
   final String _loginTime = DateFormat('HH:mm:ss').format(DateTime.now());
   bool _forceDialogShown = false;
+  bool _licenseDialogShown = false;
   final GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();
 
   void _showForceUpdateDialog(
@@ -157,6 +159,7 @@ class _AppShellState extends ConsumerState<AppShell> {
     final updateInfo = ref.watch(updateProvider).valueOrNull;
     final appVersion =
         ref.watch(appVersionProvider).valueOrNull ?? 'v1.0.0';
+    final licenseInfo = ref.watch(licenseProvider).valueOrNull;
 
     return SelectionArea(
       child: Scaffold(
@@ -350,6 +353,23 @@ class _AppShellState extends ConsumerState<AppShell> {
                           });
                           return const SizedBox.shrink();
                         }),
+                      // License expiry banner
+                      if (licenseInfo != null &&
+                          licenseInfo.needsAttention)
+                        _buildLicenseBanner(licenseInfo),
+                      // License expiry dialog (once per session)
+                      if (licenseInfo != null &&
+                          licenseInfo.needsAttention &&
+                          !_licenseDialogShown)
+                        Builder(builder: (ctx) {
+                          WidgetsBinding.instance.addPostFrameCallback((_) {
+                            if (!_licenseDialogShown && mounted) {
+                              _licenseDialogShown = true;
+                              _showLicenseExpiryDialog(ctx, licenseInfo);
+                            }
+                          });
+                          return const SizedBox.shrink();
+                        }),
                       // Offline banner
                       if (!isOnline)
                         Container(
@@ -559,6 +579,91 @@ class _StatusItem extends StatelessWidget {
       ],
     );
   }
+
+  Widget _buildLicenseBanner(LicenseInfo lic) {
+    final Color bg;
+    final String msg;
+    switch (lic.phase) {
+      case 'locked':
+        bg = AppTheme.danger;
+        msg = '授权已锁定，所有写操作已停用 — 请立即续费或激活新授权码';
+      case 'readonly':
+        bg = const Color(0xFFB71C1C);
+        msg = '授权已过期，当前为只读模式（剩余宽限期 ${lic.daysRemaining ?? 0} 天后彻底锁定）';
+      default: // grace
+        bg = const Color(0xFFE65100);
+        msg = '授权将于 ${lic.daysRemaining ?? 0} 天后到期，请及时续费';
+    }
+    return Container(
+      width: double.infinity,
+      color: bg,
+      padding: const EdgeInsets.symmetric(horizontal: 16, vertical: 6),
+      child: Row(
+        children: [
+          const Icon(Icons.warning_amber_rounded,
+              size: 16, color: Colors.white),
+          const SizedBox(width: 8),
+          Expanded(
+            child: Text(msg,
+                style: const TextStyle(color: Colors.white, fontSize: 13),
+                overflow: TextOverflow.ellipsis),
+          ),
+          TextButton(
+            onPressed: () => context.go('/settings'),
+            style:
+                TextButton.styleFrom(foregroundColor: Colors.white70),
+            child: const Text('去激活'),
+          ),
+        ],
+      ),
+    );
+  }
+
+  void _showLicenseExpiryDialog(BuildContext ctx, LicenseInfo lic) {
+    final String title;
+    final String body;
+    final Color titleColor;
+    switch (lic.phase) {
+      case 'locked':
+        title = '授权已锁定';
+        body = '您的授权已到期超过 15 天，所有写操作已停用。\n请前往「设置 → 授权」激活新的授权码，或联系客服续费。';
+        titleColor = AppTheme.danger;
+      case 'readonly':
+        title = '授权已过期 · 只读模式';
+        body = '您的授权已过期，系统进入只读模式，无法执行任何写操作。\n到期 15 天后将彻底锁定登录，请尽快续费。';
+        titleColor = AppTheme.danger;
+      default: // grace
+        title = '授权即将到期';
+        body = '您的授权将在 ${lic.daysRemaining ?? 0} 天后到期，到期后系统进入只读模式。\n请提前联系客服续费，避免影响正常使用。';
+        titleColor = Colors.orange[800]!;
+    }
+    showDialog(
+      context: ctx,
+      barrierDismissible: true,
+      builder: (_) => AlertDialog(
+        title: Row(children: [
+          Icon(Icons.warning_amber_rounded, color: titleColor, size: 20),
+          const SizedBox(width: 8),
+          Text(title, style: TextStyle(color: titleColor, fontSize: 16)),
+        ]),
+        content: Text(body, style: const TextStyle(fontSize: 14)),
+        actions: [
+          TextButton(
+            onPressed: () => Navigator.pop(_),
+            child: const Text('稍后处理'),
+          ),
+          ElevatedButton(
+            style: ElevatedButton.styleFrom(backgroundColor: titleColor),
+            onPressed: () {
+              Navigator.pop(_);
+              ctx.go('/settings');
+            },
+            child: const Text('立即前往', style: TextStyle(color: Colors.white)),
+          ),
+        ],
+      ),
+    );
+  }
 }
 
 class _StatusDivider extends StatelessWidget {