fix: resolve 3 timeline UI bugs (blank start, spinner style, tool status)

1. 任务开始时空白状态：当 agent streaming 启动但尚无 assistant
   响应时，在时间线末尾插入虚拟 "处理中..." 节点（带脉动 * 动画），
   避免用户发送 prompt 后界面无任何反馈。
   (chat_page.dart: _needsWorkingNode + ListView itemCount+1)

2. * 动画从旋转改为脉动：_SpinnerDot 由 Transform.rotate 改为
   Transform.scale（0.6x↔1.2x 缩放 + 透明度 0.6↔1.0 呼吸），
   duration 从 1000ms 降至 800ms 并启用 reverse，视觉效果类似
   星星闪烁而非机械旋转。
   (timeline_event_node.dart: _SpinnerDotState)

3. 工具执行完成后状态卡在 spinner：ToolResultEvent 到达时仅创建
   新 toolResult 消息，未回溯更新对应 toolUse 消息的 ToolStatus，
   导致时间线上工具节点永远显示 executing spinner。修复：在
   ToolResultEvent handler 中向前查找最近的 executing 状态的
   toolUse 消息，将其 status 更新为 completed/error。
   (chat_providers.dart: ToolResultEvent case)

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

it0_app/lib/features/chat/presentation/pages/chat_page.dart

@@ -51,6 +51,15 @@ class _ChatPageState extends ConsumerState<ChatPage> {
     );
   }
 
+  /// Whether to show a virtual "working" node at the bottom of the timeline.
+  /// True when the agent is streaming but no assistant message has appeared yet.
+  bool _needsWorkingNode(ChatState chatState) {
+    if (!chatState.isStreaming) return false;
+    if (chatState.messages.isEmpty) return false;
+    // Show working node if the last message is still the user's prompt
+    return chatState.messages.last.role == MessageRole.user;
+  }
+
   // -- Timeline node builder ------------------------------------------------
 
   Widget _buildTimelineNode(
@@ -242,14 +251,31 @@ class _ChatPageState extends ConsumerState<ChatPage> {
                 : ListView.builder(
                     controller: _scrollController,
                     padding: const EdgeInsets.fromLTRB(12, 8, 12, 8),
-                    itemCount: chatState.messages.length,
+                    // Extra item for the "working" placeholder when streaming
+                    // and the last message is still a user message (no agent
+                    // response has arrived yet).
+                    itemCount: chatState.messages.length +
+                        (_needsWorkingNode(chatState) ? 1 : 0),
                     itemBuilder: (context, index) {
+                      // Render the virtual "working" node at the end
+                      if (index == chatState.messages.length &&
+                          _needsWorkingNode(chatState)) {
+                        return TimelineEventNode(
+                          status: NodeStatus.active,
+                          label: '处理中...',
+                          isFirst: false,
+                          isLast: false,
+                        );
+                      }
+                      final isRealLast =
+                          index == chatState.messages.length - 1;
                       return _buildTimelineNode(
                         chatState.messages[index],
                         chatState,
                         isFirst: index == 0,
-                        isLast: index == chatState.messages.length - 1 &&
+                        isLast: isRealLast &&
-                            !chatState.isStreaming,
+                            !chatState.isStreaming &&
+                            !_needsWorkingNode(chatState),
                       );
                     },
                   ),

it0_app/lib/features/chat/presentation/providers/chat_providers.dart

@@ -194,6 +194,22 @@ class ChatNotifier extends StateNotifier<ChatState> {
         );
 
       case ToolResultEvent(:final toolName, :final output, :final isError):
+        // First, update the matching ToolUse message's status so its spinner
+        // transitions to a completed/error icon in the timeline.
+        final updatedMessages = [...state.messages];
+        for (int i = updatedMessages.length - 1; i >= 0; i--) {
+          final m = updatedMessages[i];
+          if (m.type == MessageType.toolUse &&
+              m.toolExecution?.status == ToolStatus.executing) {
+            updatedMessages[i] = m.copyWith(
+              toolExecution: m.toolExecution!.copyWith(
+                status: isError ? ToolStatus.error : ToolStatus.completed,
+              ),
+            );
+            break;
+          }
+        }
+
         final msg = ChatMessage(
           id: DateTime.now().microsecondsSinceEpoch.toString(),
           role: MessageRole.assistant,
@@ -208,7 +224,7 @@ class ChatNotifier extends StateNotifier<ChatState> {
             status: isError ? ToolStatus.error : ToolStatus.completed,
           ),
         );
-        state = state.copyWith(messages: [...state.messages, msg]);
+        state = state.copyWith(messages: [...updatedMessages, msg]);
 
       case ApprovalRequiredEvent(:final taskId, :final command, :final riskLevel):
         final msg = ChatMessage(

it0_app/lib/features/chat/presentation/widgets/timeline_event_node.dart

@@ -1,4 +1,3 @@
-import 'dart:math' as math;
 import 'package:flutter/material.dart';
 import '../../../../core/theme/app_colors.dart';
 
@@ -132,7 +131,7 @@ class _StaticDot extends StatelessWidget {
   }
 }
 
-/// Animated spinning asterisk for active nodes.
+/// Animated pulsing asterisk for active nodes (scale up/down like a star).
 class _SpinnerDot extends StatefulWidget {
   final Color color;
   const _SpinnerDot({required this.color});
@@ -150,8 +149,8 @@ class _SpinnerDotState extends State<_SpinnerDot>
     super.initState();
     _controller = AnimationController(
       vsync: this,
-      duration: const Duration(milliseconds: 1000),
+      duration: const Duration(milliseconds: 800),
-    )..repeat();
+    )..repeat(reverse: true);
   }
 
   @override
@@ -165,12 +164,14 @@ class _SpinnerDotState extends State<_SpinnerDot>
     return AnimatedBuilder(
       animation: _controller,
       builder: (_, __) {
-        return Transform.rotate(
+        // Pulse between 0.6x and 1.2x scale
-          angle: _controller.value * 2 * math.pi,
+        final scale = 0.6 + _controller.value * 0.6;
+        return Transform.scale(
+          scale: scale,
           child: Text(
             '*',
             style: TextStyle(
-              color: widget.color,
+              color: widget.color.withValues(alpha: 0.6 + _controller.value * 0.4),
               fontSize: 18,
               fontWeight: FontWeight.bold,
               height: 1,