feat(client): P5 连接页实时速度 + 连接错误冒泡 UI(#6 6E,并入 #4)

- vpnStatsProvider:暴露内核 statsStream(上/下行瞬时速率);连接页速度行
  下/上行改用实时数据(B/s→Mb/s),不再硬编码 86.4/12.1;延迟用实测节点 ping。
- ConnectionState 加 error 字段;_connect 失败不再静默回 off,而是按语言
  冒泡 ConnectApiException 文案;节点未就绪(无 uuid)给出提示并删除旧 1.2s
  mock 连接路径。
- 连接页 caption 下/桌面 pill 上方显示错误文案(红)。
- 重写 connection_controller_test 适配(无网络:未就绪→off+error)。

flutter analyze 0 error;114 tests passed。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/lib/screens/connect_page.dart

@@ -31,6 +31,9 @@ class ConnectPage extends ConsumerWidget {
     final smart = ref.watch(isSmartSelectProvider);
     final isFree = ref.watch(isFreePlanProvider);
     final quota = ref.watch(quotaProvider);
+    final stats = ref.watch(vpnStatsProvider).valueOrNull;
+    final down = _mbps(stats?.downloadSpeed);
+    final up = _mbps(stats?.uploadSpeed);
 
     final caption = switch (conn.phase) {
       VpnPhase.off => t.capOff,
@@ -46,13 +49,23 @@ class ConnectPage extends ConsumerWidget {
       onTap: () => ref.read(connectionProvider.notifier).toggle(),
     );
 
-    final captionWidget = Text(caption,
-        style: PangolinText.body.copyWith(color: c.fg2, fontWeight: FontWeight.w600));
+    final captionWidget = Column(mainAxisSize: MainAxisSize.min, children: [
+      Text(caption, style: PangolinText.body.copyWith(color: c.fg2, fontWeight: FontWeight.w600)),
+      if (conn.error != null) ...[
+        const SizedBox(height: 8),
+        Padding(
+          padding: const EdgeInsets.symmetric(horizontal: 24),
+          child: Text(conn.error!,
+              textAlign: TextAlign.center,
+              style: PangolinText.caption.copyWith(color: c.danger, fontWeight: FontWeight.w500)),
+        ),
+      ],
+    ]);
 
     final infoChildren = <Widget>[
       if (isFree)
         QuotaCard(quota: quota, t: t, onWatchAd: () => ref.read(quotaProvider.notifier).watchAd()),
-      if (conn.phase == VpnPhase.on) _SpeedRow(t: t, node: node),
+      if (conn.phase == VpnPhase.on) _SpeedRow(t: t, node: node, down: down, up: up),
       _CurrentNodeCard(t: t, node: node, smart: smart, showLabel: isWide, onTap: onOpenNodes),
     ];
 
@@ -83,6 +96,14 @@ class ConnectPage extends ConsumerWidget {
                 textAlign: TextAlign.center,
                 style: PangolinText.display.copyWith(color: c.fg1, fontSize: 22, fontWeight: FontWeight.w700)),
             const SizedBox(height: 10),
+            if (conn.error != null) ...[
+              Padding(
+                padding: const EdgeInsets.only(bottom: 10),
+                child: Text(conn.error!,
+                    textAlign: TextAlign.center,
+                    style: PangolinText.caption.copyWith(color: c.danger, fontWeight: FontWeight.w500)),
+              ),
+            ],
             _NodePill(t: t, node: node, smart: smart),
             if (isFree) ...[
               const SizedBox(height: 24),
@@ -93,7 +114,7 @@ class ConnectPage extends ConsumerWidget {
             ],
             if (conn.phase == VpnPhase.on) ...[
               const SizedBox(height: 16),
-              _SpeedRow(t: t, node: node),
+              _SpeedRow(t: t, node: node, down: down, up: up),
             ],
           ]),
         ),
@@ -174,18 +195,24 @@ class ConnectPage extends ConsumerWidget {
   }
 }
 
-/// 实时速率行(连接成功时显示;演示值)。
+/// 字节/秒 → Mb/s 字符串(null = 未取到,显示 —)。
+String _mbps(double? bytesPerSec) =>
+    bytesPerSec == null ? '—' : (bytesPerSec * 8 / 1000000).toStringAsFixed(1);
+
+/// 实时速率行(连接成功时显示;来自内核 statsStream 实时数据)。
 class _SpeedRow extends StatelessWidget {
-  const _SpeedRow({required this.t, required this.node});
+  const _SpeedRow({required this.t, required this.node, required this.down, required this.up});
   final AppText t;
   final Node node;
+  final String down;
+  final String up;
 
   @override
   Widget build(BuildContext context) {
     final c = context.pangolin;
     final metrics = [
-      (PangolinIcons.arrowDown, t.download, '86.4', 'Mb/s'),
-      (PangolinIcons.arrowUp, t.upload, '12.1', 'Mb/s'),
+      (PangolinIcons.arrowDown, t.download, down, 'Mb/s'),
+      (PangolinIcons.arrowUp, t.upload, up, 'Mb/s'),
       (PangolinIcons.zap, t.latency, node.ping > 0 ? '${node.ping}' : '—', 'ms'),
     ];
     return Row(children: [