feat(workspace): implement comprehensive keyboard navigation and multi-cell range selection in DataGrid

[ZhuchkaTriplesix]

Motivation & Problem

In lib/features/workspace/result_grid_view.dart (lines 780–886), the DataGrid handles clipboard copy (Ctrl+C), staging shortcuts (Ctrl+Del, Ctrl+Z), and inline editing triggers (F2, Enter, Space), but lacks keyboard navigation for moving the active cell selection:

• Pressing ArrowUp, ArrowDown, ArrowLeft, or ArrowRight does not move cell selection.
• Shift + Arrow keys do not extend the rectangular cell selection range.
• Ctrl + A / Meta + A does not select all cells in the grid.
• Home, End, Ctrl+Home, Ctrl+End, PageUp, PageDown are unhandled.

Proposed Solution

1. In ResultGridView (_VirtualResultGridState):
   • Add _navigateCell(int dRow, int dCol, {bool extendSelection = false}) supporting arrow keys and shift-arrow range extension.
   • Add Home / End (first/last column), Ctrl+Home / Ctrl+End (first/last cell), and PageUp / PageDown (viewport page step).
   • Add Ctrl+A / Meta+A to select the entire table grid.
   • Implement auto-scroll to ensure newly selected cells are brought into visible viewport.
2. Add widget tests for keyboard navigation, range selection, and shortcuts.

Acceptance Criteria

• Arrow keys navigate between cells smoothly.
• Shift + Arrow keys create and expand multi-cell rectangular selections.
• Ctrl+A selects all rows and columns.
• Active cell scrolls into view when navigating outside visible area.
• Unit and widget tests pass with 0 analyze warnings.