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

backend/src/modules/dc/tool-c/controllers/SessionController.ts

@@ -392,7 +392,7 @@ export class SessionController {
       // 1. 获取完整数据
       const data = await sessionService.getFullData(id);
 
-      // 2. 提取唯一值（去除空值和首尾空格）
+      // 2. 提取唯一值（保留NA值，但清理字符串）
       const values = data.map((row) => row[column]);
       const cleanedValues = values.map((val) => {
         if (val === null || val === undefined || val === '') return null;
@@ -400,12 +400,22 @@ export class SessionController {
         return typeof val === 'string' ? val.trim() : val;
       });
 
-      // 3. 去重
-      const uniqueValues = Array.from(new Set(cleanedValues))
+      // 3. 去重（✨ 保留null值，但用特殊标记表示）
+      const uniqueSet = Array.from(new Set(cleanedValues));
+      const hasNA = uniqueSet.includes(null);
+      const naCount = cleanedValues.filter(v => v === null).length;  // ✨ 统计NA数量
+      
+      // 过滤掉null和空白，然后排序
+      const nonNAValues = uniqueSet
         .filter((v) => v !== null && v !== '' && v !== '(空白)')
         .sort(); // 排序，方便查看
+      
+      // ✨ 如果有NA，添加到数组末尾（用特殊字符串标记）
+      const uniqueValues = hasNA 
+        ? [...nonNAValues, '<空值/NA>']
+        : nonNAValues;
 
-      logger.info(`[SessionController] 唯一值数量: ${uniqueValues.length}`);
+      logger.info(`[SessionController] 唯一值数量: ${uniqueValues.length} (含NA: ${hasNA}, NA数量: ${naCount})`);
 
       // 4. 返回结果
       return reply.send({
@@ -414,6 +424,8 @@ export class SessionController {
           column,
           uniqueValues,
           count: uniqueValues.length,
+          naCount: hasNA ? naCount : 0,  // ✨ 返回NA数量
+          totalCount: data.length,  // ✨ 总行数
         },
       });
     } catch (error: any) {