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/state/connection_provider.dart

@@ -11,7 +11,9 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../bridge/vpn_bridge.dart';
 import '../bridge/vpn_bridge_provider.dart';
+import '../l10n/app_text.dart';
 import '../services/connect_api.dart';
+import 'app_providers.dart';
 import 'auth_provider.dart';
 import 'nodes_provider.dart';
 
@@ -37,20 +39,26 @@ enum VpnPhase { off, connecting, on }
 // ── 连接状态快照 ──────────────────────────────────────────────────
 
 class ConnectionState {
-  const ConnectionState({required this.phase, this.elapsed = Duration.zero});
+  const ConnectionState({required this.phase, this.elapsed = Duration.zero, this.error});
 
   final VpnPhase phase;
   final Duration elapsed;
 
+  /// 连接失败原因(已本地化);null = 无错误。供 UI 提示,不再静默吞掉。
+  final String? error;
+
   ConnectionState copyWith({VpnPhase? phase, Duration? elapsed}) =>
       ConnectionState(phase: phase ?? this.phase, elapsed: elapsed ?? this.elapsed);
 
   @override
   bool operator ==(Object other) =>
-      other is ConnectionState && other.phase == phase && other.elapsed == elapsed;
+      other is ConnectionState &&
+      other.phase == phase &&
+      other.elapsed == elapsed &&
+      other.error == error;
 
   @override
-  int get hashCode => Object.hash(phase, elapsed);
+  int get hashCode => Object.hash(phase, elapsed, error);
 }
 
 // ── 状态机 ───────────────────────────────────────────────────────
@@ -94,15 +102,18 @@ class ConnectionController extends StateNotifier<ConnectionState> {
   Future<void> _connect() async {
     state = const ConnectionState(phase: VpnPhase.connecting);
 
-    final authState = _ref.read(authProvider);
-    final token = authState.accessToken ?? '';
+    final token = _ref.read(authProvider).accessToken ?? '';
     final node = _ref.read(effectiveNodeProvider);
+    final zh = _ref.read(localeProvider) == AppLang.zh;
 
-    // 无 UUID 时（演示节点）直接进入 mock 连接状态
+    // 节点未就绪(列表加载中/为空):不连接,提示用户。
     if (node.uuid.isEmpty) {
-      await Future<void>.delayed(const Duration(milliseconds: 1200));
-      if (mounted) state = const ConnectionState(phase: VpnPhase.on);
-      _startElapsed();
+      if (mounted) {
+        state = ConnectionState(
+          phase: VpnPhase.off,
+          error: zh ? '节点尚未就绪,请稍候重试' : 'Nodes not ready, please retry',
+        );
+      }
       return;
     }
 
@@ -115,8 +126,16 @@ class ConnectionController extends StateNotifier<ConnectionState> {
       );
       // bridge.start() 不阻塞至连接建立；on 状态由 statusStream 回调驱动。
       await _bridge.start(configJson);
+    } on ConnectApiException catch (e) {
+      // 把后端/网络错误冒泡到 UI(原静默回 off,用户不知所以)。
+      if (mounted) state = ConnectionState(phase: VpnPhase.off, error: zh ? e.messageZh : e.messageEn);
     } catch (e) {
-      if (mounted) state = const ConnectionState(phase: VpnPhase.off);
+      if (mounted) {
+        state = ConnectionState(
+          phase: VpnPhase.off,
+          error: zh ? '连接失败,请重试' : 'Connection failed, please retry',
+        );
+      }
     }
   }
 
@@ -173,3 +192,8 @@ final connectionProvider =
     StateNotifierProvider<ConnectionController, ConnectionState>(
   (ref) => ConnectionController(ref, ref.watch(vpnBridgeProvider)),
 );
+
+/// 内核实时统计流(上/下行瞬时速率、字节数)。连接页速度行的真实数据源。
+final vpnStatsProvider = StreamProvider<VpnStatsEvent>(
+  (ref) => ref.watch(vpnBridgeProvider).statsStream,
+);