Excel-style footer with dynamic column aggregations

[SabaliDev]

Summary

Implements Excel-style footer with dynamic column aggregations using SlickGrid's native footer API.

Changes

• Added dedicated "Total Row" footer with dropdown menus for 7 aggregation types (Sum, Avg, Count, Min, Max, Unique, None)
• Supports real-time updates and independent per-column customization
• Leverages SlickGrid's built-in footer API for proper event handling and styling

Technical Decision

Chosen approach: SlickGrid built-in footer API

• Full interactive functionality (dropdowns, click events)
• Maintainable and extensible architecture
• Proper event handling and cell isolation
• Footer renders under header, before data rows

Rejected approach: Last data row as footer

• SlickGrid data row events don't support UI controls
• Cannot handle dropdown interactions or independent cell events
• Would position at bottom (after data rows)

Rationale

SlickGrid's data row model isn't designed for embedded interactive controls. The footer API provides independent cell access and event handling, making it the only viable path for true Excel-like functionality without major architectural changes.

Known Limitations

• Footer appears above data rows rather than at the bottom (inherent to SlickGrid's footer positioning)
• This deviates from traditional Excel table layout but maintains full functionality