feat(app): 全局离线提示横幅 — 网络不可用时显示黄色警告

- 新增 OfflineBanner widget: 监听 connectivity_plus 自动显示/隐藏
- AnimatedCrossFade 滑入滑出动画 + warmCurve 弹性曲线
- 黄色警告条: wifi_off 图标 + '网络不可用，部分功能受限'
- 嵌入 ResponsiveScaffold 的 body 上方 (手机/平板/桌面三端)
- 只在离线时显示，恢复网络后自动消失

app/lib/widgets/offline_banner.dart

@@ -0,0 +1,102 @@
+// 全局离线提示横幅 — 监听 connectivity_plus 显示/隐藏
+//
+// 放在 Scaffold body 上方，离线时显示黄色警告横幅
+// 使用: 在 responsive_scaffold 的 body 上方嵌套
+
+import 'package:connectivity_plus/connectivity_plus.dart';
+import 'package:flutter/material.dart';
+
+import '../core/constants/design_tokens.dart';
+import '../core/theme/app_colors.dart';
+
+/// 全局离线提示横幅 — 自动监听网络状态
+class OfflineBanner extends StatefulWidget {
+  const OfflineBanner({super.key, required this.child});
+
+  final Widget child;
+
+  @override
+  State<OfflineBanner> createState() => _OfflineBannerState();
+}
+
+class _OfflineBannerState extends State<OfflineBanner> {
+  bool _isOffline = false;
+
+  @override
+  void initState() {
+    super.initState();
+    // 初始检查
+    Connectivity().checkConnectivity().then((result) {
+      if (mounted) {
+        setState(() {
+          _isOffline = result.every((r) => r == ConnectivityResult.none);
+        });
+      }
+    });
+    // 监听变化
+    Connectivity().onConnectivityChanged.listen((result) {
+      if (mounted) {
+        final offline = result.every((r) => r == ConnectivityResult.none);
+        if (offline != _isOffline) {
+          setState(() => _isOffline = offline);
+        }
+      }
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Column(
+      children: [
+        // 离线横幅 — 带动画滑入/滑出
+        AnimatedCrossFade(
+          firstChild: const SizedBox.shrink(),
+          secondChild: _OfflineBar(),
+          crossFadeState: _isOffline
+              ? CrossFadeState.showSecond
+              : CrossFadeState.showFirst,
+          duration: DesignTokens.animNormal,
+          sizeCurve: DesignTokens.warmCurve,
+        ),
+        // 正常内容
+        Expanded(child: widget.child),
+      ],
+    );
+  }
+}
+
+/// 离线横幅条
+class _OfflineBar extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return Container(
+      width: double.infinity,
+      padding: const EdgeInsets.symmetric(horizontal: 16, vertical: 8),
+      decoration: BoxDecoration(
+        color: AppColors.warning.withValues(alpha: 0.15),
+        border: Border(
+          bottom: BorderSide(
+            color: AppColors.warning.withValues(alpha: 0.3),
+            width: 1,
+          ),
+        ),
+      ),
+      child: Row(
+        children: [
+          Icon(Icons.wifi_off_rounded, size: 16, color: AppColors.warning),
+          const SizedBox(width: 8),
+          Expanded(
+            child: Text(
+              '网络不可用，部分功能受限',
+              style: TextStyle(
+                fontSize: 13,
+                color: AppColors.warning,
+                fontWeight: FontWeight.w500,
+              ),
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}

app/lib/widgets/responsive_scaffold.dart

@@ -9,6 +9,7 @@ import '../core/constants/breakpoints.dart';
 import '../core/theme/app_colors.dart';
 import '../core/theme/app_typography.dart';
 import '../core/theme/app_radius.dart';
+import 'offline_banner.dart';
 
 /// 导航项数量（不含中心 FAB）
 const int kNavItemCount = 4;
@@ -167,7 +168,7 @@ class _MobileLayout extends StatelessWidget {
       appBar: appBarTitle != null
           ? AppBar(title: Text(appBarTitle!))
           : null,
-      body: body,
+      body: OfflineBanner(child: body),
       extendBody: true, // 允许内容延伸到 Tab 栏下面（圆角透明效果）
       bottomNavigationBar: _BottomNavBar(
         selectedIndex: selectedIndex,