feat(device): 启动自动连接 USB 串口 + 隐藏设置页配置项 + 标题栏连接状态

- 新增 AutoSerialConnect 服务：启动后自动连接第一个 USB 串口设备，
  固定 115200/8/N/1，连接失败时每 3s 重试，断开后重新进入重试循环
- main.dart 通过 ProviderContainer 在 runApp 之前触发 autoSerialConnectProvider
- 移除设置页「串口配置」菜单项及对应面板分支
- StatusBar 在「设备运行状态」前增加串口连接状态指示（已连接/连接中/未连接）

lib/features/home/widgets/status_bar.dart

@@ -5,6 +5,8 @@ import '../../../core/localization/app_localizations.dart';
 import '../../../core/theme/app_theme.dart';
 import '../../../shared/widgets/status_indicator.dart';
 import '../../device/providers/device_info_provider.dart';
+import '../../device/providers/serial_provider.dart';
+import '../../device/services/serial_port_service.dart';
 
 /// 状态栏标签项数据
 class StatusBarTab {
@@ -76,6 +78,7 @@ class _StatusBarState extends ConsumerState<StatusBar> {
   Widget build(BuildContext context) {
     final l10n = AppLocalizations.of(context);
     final deviceInfo = ref.watch(deviceInfoProvider);
+    final serialState = ref.watch(serialPortServiceProvider).state;
 
     return Container(
       height: 56,
@@ -117,6 +120,8 @@ class _StatusBarState extends ConsumerState<StatusBar> {
             onTap: _onLightTap,
           ),
           const SizedBox(width: 20),
+          _SerialConnectionIndicator(state: serialState),
+          const SizedBox(width: 20),
           StatusIndicator(
             text: widget.isRunning
                 ? (l10n?.running ?? '运行中')
@@ -262,3 +267,49 @@ class _LightToggleButton extends StatelessWidget {
     );
   }
 }
+
+/// 串口连接状态指示器
+///
+/// 位于「设备运行状态」之前，反映当前 USB 串口的连接情况。
+class _SerialConnectionIndicator extends StatelessWidget {
+  final SerialConnectionState state;
+  const _SerialConnectionIndicator({required this.state});
+
+  @override
+  Widget build(BuildContext context) {
+    final connected = state == SerialConnectionState.connected;
+    final connecting = state == SerialConnectionState.connecting;
+    final text = connected
+        ? '已连接'
+        : connecting
+            ? '连接中'
+            : '未连接';
+    final color = connected
+        ? AppTheme.statusRunning
+        : connecting
+            ? AppTheme.statusPaused
+            : AppTheme.statusStopped;
+    return Row(
+      mainAxisSize: MainAxisSize.min,
+      children: [
+        Container(
+          width: 10,
+          height: 10,
+          decoration: BoxDecoration(
+            shape: BoxShape.circle,
+            color: color,
+          ),
+        ),
+        const SizedBox(width: 6),
+        Text(
+          text,
+          style: TextStyle(
+            color: Colors.white,
+            fontWeight: FontWeight.w600,
+            fontSize: 13,
+          ),
+        ),
+      ],
+    );
+  }
+}