fix: 彻底修复 TDZ 错误和用户消息不显示问题

1. agent-engine.ts: onNewIteration 改用 prevToolCalls 传递上一轮工具调用，
   避免引用未声明的 toolCalls 导致 TDZ (Cannot access 'B' before initialization)
2. chat-area.ts: 新增 resetCurrentPlaceholder() 清理悬空 DOM 引用
3. input-area.ts: 错误处理器删除 placeholder 后清理 currentPlaceholder 引用，
   防止后续 updateLastAssistantMessage 操作已脱离 DOM 的旧节点

src/renderer/components/chat-area.ts

@@ -495,6 +495,13 @@ export function clearMessages(): void {
   currentPlaceholder = null;
 }
 
+/** 清理 currentPlaceholder 引用（当 placeholder 从外部被移除时调用） */
+export function resetCurrentPlaceholder(): void {
+  if (currentPlaceholder && !currentPlaceholder.isConnected) {
+    currentPlaceholder = null;
+  }
+}
+
 export function enableAutoScroll(): void {
   autoScroll = true;
   if (scrollBtnEl) scrollBtnEl.style.display = 'none';

src/renderer/components/input-area.ts

@@ -9,7 +9,7 @@ import { getSelectedModel, isThinkEnabled, isVisionAvailable, isToolCallingSuppo
 import {
   renderMessages, clearMessagesDOM, appendAssistantPlaceholder, appendSystemMessage,
   updateLastAssistantMessage, clearMessages, safeMarkdown, enableAutoScroll, updateTotalTokens,
-  appendToolCallCardToPlaceholder, updateToolCallCardInPlaceholder
+  appendToolCallCardToPlaceholder, updateToolCallCardInPlaceholder, resetCurrentPlaceholder
 } from './chat-area.js';
 import { showToast } from './toast.js';
 import { addToolCard, updateToolCard, clearToolCardsExternal, clearTerminalExternal } from './workspace-panel.js';
@@ -1109,8 +1109,10 @@ async function sendMessageWithAgentLoop(text: string, currentSession: ChatSessio
         placeholder.classList.remove('loading');
         placeholder.querySelector('.loading-dots')?.remove();
         placeholder.querySelector('.loading-text')?.remove();
-        if (!assistantContent) placeholder.remove();
-        else {
+        if (!assistantContent) {
+          placeholder.remove();
+          resetCurrentPlaceholder();
+        } else {
           const contentDiv = placeholder.querySelector('.msg-content');
           if (contentDiv) contentDiv.innerHTML = safeMarkdown(assistantContent) + '<p><code>[已中断]</code></p>';
         }

src/renderer/services/agent-engine.ts

@@ -513,8 +513,8 @@ export async function runAgentLoop(
   let totalEvalCount = 0;
   /** 跨轮去重：仅跟踪成功的工具调用（失败的允许重试） */
   let prevLoopSuccessKeys: string[] = [];
-  /** 每轮工具调用计数（用于去重） */
-  let allCallsThisLoop: ToolCall[] = [];
+  /** 保存上一轮的工具调用，供 onNewIteration 使用 */
+  let prevToolCalls: ToolCall[] = [];
 
   const makeStats = () => {
     const totalDuration = (Date.now() - loopStartTime) * 1e6;
@@ -535,7 +535,7 @@ export async function runAgentLoop(
 
     // 非首轮迭代：通知 UI 创建新的消息气泡（防止每轮内容互相覆盖）
     if (loopCount > 1 && callbacks.onNewIteration) {
-      callbacks.onNewIteration(toolCalls.length > 0 ? toolCalls : undefined);
+      callbacks.onNewIteration(prevToolCalls.length > 0 ? prevToolCalls : undefined);
     }
 
     logAgentLoop(loopCount, maxLoops);
@@ -543,7 +543,6 @@ export async function runAgentLoop(
     let thinking = '';
     content = '';
     const toolCalls: ToolCall[] = [];
-    allCallsThisLoop = [];
 
     // v5.1.2 预算警告：接近迭代上限时注入临时提示
     let budgetWarningIdx = -1;
@@ -873,6 +872,9 @@ export async function runAgentLoop(
       `${r.name}: ${r.result?.success ? 'success' : 'error'}`
     ).join('; ');
     saveTrace(traceStep);
+
+    // 保存本轮工具调用，供下一轮 onNewIteration 使用
+    prevToolCalls = toolCalls;
   }
 
   logWarn('ReAct Agent Loop 达到最大工具调用次数限制');