fix: improve voice call page UI centering and error display

1. 布局居中：将 Column 包裹在 Center 中，所有文本添加
   textAlign: TextAlign.center，确保头像、标题、副标题
   在各种屏幕尺寸上居中显示。

2. 错误展示优化：将 SnackBar 大面积红色块替换为行内错误卡片，
   采用圆角容器 + error icon + 简洁文案，视觉上更融洽。
   新增 _errorMessage 状态字段 + _friendlyError() 方法，
   将 DioException 等异常转换为中文友好提示（如 "语音服务
   暂不可用 (503)"），避免用户看到大段英文 stacktrace。

3. 错误状态清理：点击接听时自动清除上一次的 _errorMessage。

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

it0_app/lib/features/agent_call/presentation/pages/agent_call_page.dart

@@ -26,6 +26,7 @@ class AgentCallPage extends ConsumerStatefulWidget {
 class _AgentCallPageState extends ConsumerState<AgentCallPage>
     with SingleTickerProviderStateMixin {
   _CallPhase _phase = _CallPhase.ringing;
+  String? _errorMessage;
   String? _sessionId;
   WebSocketChannel? _audioChannel;
   StreamSubscription? _audioSubscription;
@@ -77,7 +78,10 @@ class _AgentCallPageState extends ConsumerState<AgentCallPage>
 
   /// Accept call: create voice session, connect WebSocket, start mic + player.
   Future<void> _acceptCall() async {
-    setState(() => _phase = _CallPhase.connecting);
+    setState(() {
+      _phase = _CallPhase.connecting;
+      _errorMessage = null;
+    });
 
     try {
       final dio = ref.read(dioClientProvider);
@@ -136,13 +140,10 @@ class _AgentCallPageState extends ConsumerState<AgentCallPage>
       setState(() => _phase = _CallPhase.active);
     } catch (e) {
       if (mounted) {
-        ScaffoldMessenger.of(context).showSnackBar(
+        setState(() {
-          SnackBar(
+          _phase = _CallPhase.ringing;
-            content: Text('通话连接失败: $e'),
+          _errorMessage = '连接失败: ${_friendlyError(e)}';
-            backgroundColor: AppColors.error,
+        });
-          ),
-        );
-        setState(() => _phase = _CallPhase.ringing);
       }
     }
   }
@@ -434,22 +435,60 @@ class _AgentCallPageState extends ConsumerState<AgentCallPage>
       body: SafeArea(
         child: Stack(
           children: [
-            Column(
+            Center(
+              child: Column(
                 children: [
                   const Spacer(flex: 2),
                   _buildAvatar(),
                   const SizedBox(height: 24),
                   Text(
                     _statusText,
+                    textAlign: TextAlign.center,
                     style: const TextStyle(
                         fontSize: 24, fontWeight: FontWeight.bold),
                   ),
                   const SizedBox(height: 8),
                   Text(
                     _subtitleText,
+                    textAlign: TextAlign.center,
                     style: const TextStyle(
                         color: AppColors.textSecondary, fontSize: 15),
                   ),
+                  // Inline error message
+                  if (_errorMessage != null) ...[
+                    const SizedBox(height: 16),
+                    Padding(
+                      padding: const EdgeInsets.symmetric(horizontal: 32),
+                      child: Container(
+                        padding: const EdgeInsets.symmetric(
+                            horizontal: 16, vertical: 12),
+                        decoration: BoxDecoration(
+                          color: AppColors.error.withOpacity(0.1),
+                          borderRadius: BorderRadius.circular(12),
+                          border: Border.all(
+                              color: AppColors.error.withOpacity(0.3)),
+                        ),
+                        child: Row(
+                          mainAxisSize: MainAxisSize.min,
+                          children: [
+                            const Icon(Icons.error_outline,
+                                size: 18, color: AppColors.error),
+                            const SizedBox(width: 8),
+                            Flexible(
+                              child: Text(
+                                _errorMessage!,
+                                textAlign: TextAlign.center,
+                                style: const TextStyle(
+                                  color: AppColors.error,
+                                  fontSize: 13,
+                                ),
+                              ),
+                            ),
+                          ],
+                        ),
+                      ),
+                    ),
+                  ],
                   const SizedBox(height: 32),
                   if (_phase == _CallPhase.active)
                     Text(
@@ -468,6 +507,7 @@ class _AgentCallPageState extends ConsumerState<AgentCallPage>
                   const SizedBox(height: 48),
                 ],
               ),
+            ),
             // Reconnecting overlay
             if (_isReconnecting)
               Positioned(
@@ -522,6 +562,9 @@ class _AgentCallPageState extends ConsumerState<AgentCallPage>
   }
 
   String get _subtitleText {
+    if (_errorMessage != null && _phase == _CallPhase.ringing) {
+      return '';
+    }
     switch (_phase) {
       case _CallPhase.ringing:
         return '智能运维语音助手';
@@ -534,6 +577,31 @@ class _AgentCallPageState extends ConsumerState<AgentCallPage>
     }
   }
 
+  /// Extract a short user-friendly message from an exception.
+  String _friendlyError(dynamic e) {
+    final s = e.toString();
+    // Extract status code info for Dio errors
+    final match = RegExp(r'status code of (\d+)').firstMatch(s);
+    if (match != null) {
+      final code = match.group(1);
+      switch (code) {
+        case '503':
+          return '语音服务暂不可用 ($code)';
+        case '502':
+          return '语音服务网关错误 ($code)';
+        case '404':
+          return '语音服务未找到 ($code)';
+        default:
+          return '服务器错误 ($code)';
+      }
+    }
+    if (s.contains('SocketException') || s.contains('Connection refused')) {
+      return '无法连接到服务器';
+    }
+    if (s.length > 80) return '${s.substring(0, 80)}...';
+    return s;
+  }
+
   Widget _buildAvatar() {
     final isActive = _phase == _CallPhase.active;
     return AnimatedContainer(