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
HaHafeng
f4f1d09837