perf(ui): 2D virtualization for VirtualResultGrid (wide columns)

[ZhuchkaTriplesix]

Parent

Part of #414

Problem

VirtualResultGrid virtualizes rows (ListView.builder + itemExtent) but each row still builds a full Row of all columns. Wide result sets (100+ cols) still build O(visibleRows × cols) cells. Horizontal scroll wraps the entire vertical list.

Evidence

• lib/shared/widgets/result_grid_view.dart (or equivalent VirtualResultGrid path)

Acceptance

• Off-screen columns are not built (or cells are recycled)
• Scroll remains smooth for wide grids (document target: ≥50 cols × 1k visible-capable rows)
• Header stays aligned with body columns

Suggested fix

Two-axis viewport / TableView / recycle cells; optional freeze first column.

[ZhuchkaTriplesix]

Shipped on dev via #444 (Closes did not auto-close because PRs targeted dev).