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

extraction_service/operations/conditional.py

@@ -109,6 +109,10 @@ def apply_conditional_column(
                 mask = col_data >= value
             elif operator == '<=':
                 mask = col_data <= value
+            elif operator == 'is_null':  # ✨ 新增：为空
+                mask = result[column].isna()
+            elif operator == 'not_null':  # ✨ 新增：不为空
+                mask = result[column].notna()
             else:
                 raise ValueError(f'不支持的运算符: {operator}')