feat(dc/tool-c): Add pivot column ordering and NA handling features

Major features:
1. Pivot transformation enhancements:
   - Add option to keep unselected columns with 3 aggregation methods
   - Maintain original column order after pivot (aligned with source file)
   - Preserve pivot value order (first appearance order)

2. NA handling across 4 core functions:
   - Recode: Support keep/map/drop for NA values
   - Filter: Already supports is_null/not_null operators
   - Binning: Support keep/label/assign for NA values (fix nan display)
   - Conditional: Add is_null/not_null operators

3. UI improvements:
   - Enable column header tooltips with custom header component
   - Add closeable alert for 50-row preview
   - Fix page scrollbar issues

Modified files:
Python: pivot.py, recode.py, binning.py, conditional.py, main.py
Backend: SessionController, QuickActionController, QuickActionService
Frontend: PivotDialog, RecodeDialog, BinningDialog, ConditionalDialog, DataGrid, index

Status: Ready for testing

frontend-v2/src/modules/dc/pages/tool-c/index.tsx

@@ -36,6 +36,7 @@ interface ToolCState {
   // UI状态
   isLoading: boolean;
   isSidebarOpen: boolean;
+  isAlertClosed: boolean; // ✨ 新增：提示条关闭状态
   
   // ✨ 功能按钮对话框状态
   filterDialogVisible: boolean;
@@ -69,6 +70,7 @@ const ToolC = () => {
     messages: [],
     isLoading: false,
     isSidebarOpen: true,
+    isAlertClosed: false, // ✨ 初始状态：未关闭
     filterDialogVisible: false,
     recodeDialogVisible: false,
     binningDialogVisible: false,
@@ -228,7 +230,7 @@ const ToolC = () => {
 
   // ==================== 渲染 ====================
   return (
-    <div className="h-screen w-screen flex flex-col bg-gradient-to-br from-slate-50 to-slate-100 overflow-hidden">
+    <div className="h-screen w-screen flex flex-col bg-gradient-to-br from-slate-50 to-slate-100">
       {/* 顶部栏 */}
       <Header 
         fileName={state.fileName || '未上传文件'} 
@@ -237,10 +239,10 @@ const ToolC = () => {
         onToggleSidebar={() => updateState({ isSidebarOpen: !state.isSidebarOpen })}
       />
       
-      {/* 主工作区 */}
-      <div className="flex-1 flex overflow-hidden">
-        {/* 左侧：表格区域 */}
-        <div className="flex-1 flex flex-col min-w-0 overflow-hidden">
+      {/* 主工作区 - 移除overflow-hidden，让子元素自己处理滚动 */}
+      <div className="flex-1 flex min-h-0">
+        {/* 左侧：表格区域 - 独立滚动 */}
+        <div className="flex-1 flex flex-col min-w-0">
           <Toolbar 
             sessionId={state.sessionId}
             onFilterClick={() => updateState({ filterDialogVisible: true })}
@@ -251,23 +253,34 @@ const ToolC = () => {
             onComputeClick={() => updateState({ computeDialogVisible: true })}
             onPivotClick={() => updateState({ pivotDialogVisible: true })}
           />
-          <div className="flex-1 p-4 overflow-hidden flex flex-col">
-            {/* ✨ 优化：提示只显示前50行 */}
-            {state.data.length > 0 && (
-              <div className="mb-2 px-3 py-2 bg-blue-50 border border-blue-200 rounded-lg flex items-center gap-2 text-sm">
-                <span className="text-blue-600">ℹ️</span>
-                <span className="text-blue-700">
-                  <strong>提示：</strong>表格仅展示前 <strong>50行</strong> 数据预览，导出功能将包含 <strong>全部</strong> 处理结果
-                </span>
+          <div className="flex-1 p-4 flex flex-col min-h-0">
+            {/* ✨ 优化：提示只显示前50行（可关闭） */}
+            {state.data.length > 0 && !state.isAlertClosed && (
+              <div className="mb-2 px-3 py-2 bg-blue-50 border border-blue-200 rounded-lg flex items-center justify-between gap-2 text-sm">
+                <div className="flex items-center gap-2">
+                  <span className="text-blue-600">ℹ️</span>
+                  <span className="text-blue-700">
+                    <strong>提示：</strong>表格仅展示前 <strong>50行</strong> 数据预览，导出功能将包含 <strong>全部</strong> 处理结果
+                  </span>
+                </div>
+                <button
+                  onClick={() => updateState({ isAlertClosed: true })}
+                  className="text-blue-400 hover:text-blue-600 transition-colors p-1 rounded hover:bg-blue-100"
+                  title="关闭提示"
+                >
+                  <svg width="16" height="16" viewBox="0 0 16 16" fill="currentColor">
+                    <path d="M4.646 4.646a.5.5 0 0 1 .708 0L8 7.293l2.646-2.647a.5.5 0 0 1 .708.708L8.707 8l2.647 2.646a.5.5 0 0 1-.708.708L8 8.707l-2.646 2.647a.5.5 0 0 1-.708-.708L7.293 8 4.646 5.354a.5.5 0 0 1 0-.708z"/>
+                  </svg>
+                </button>
               </div>
             )}
-            <div className="flex-1 overflow-hidden">
+            <div className="flex-1 min-h-0">
               <DataGrid data={state.data} columns={state.columns} />
             </div>
           </div>
         </div>
 
-        {/* 右侧：AI 数据清洗助手 */}
+        {/* 右侧：AI 数据清洗助手 - 独立滚动 */}
         {state.isSidebarOpen && (
           <Sidebar
             isOpen={state.isSidebarOpen}