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/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>';
         }