perf(grid): hoist secondary click (context menu) and tooltip handling from _GridCell to grid level (#1346)

[ZhuchkaTriplesix]

Closes #1346

Summary of Changes:

1. Hoisted Context Menu:
   • Replaced mounting per-cell ContextMenu and Listener(onPointerDown) with grid-level secondary mouse click handling in _onGridPointerDown.
   • Context menus are opened on-demand at pointer location using exported showContextMenuListenable / showContextMenuBuilder from shadcn_flutter.
   • Items are built strictly on secondary tap, retaining lazy menu item construction count guarantees.
2. Hoisted Cell Tooltips:
   • Replaced mounting per-cell material.Tooltip with a grid-level MouseRegion hover listener and an OverlayEntry displayed with a 450 ms debounce.
   • Cleans up and dismisses tooltip on horizontal/vertical scrolling, pointer down, and hover exit.
3. Optimized _GridCell & _DataRow:
   • Removed 10+ context menu callbacks forwarded through _DataRow and _GridCell.
   • Simplified _GridCell.sameAs from 31 checked fields down to 18 fields.
   • Reduced widget tree depth by eliminating ~4200 redundant intermediate widgets per 600 visible cells.
4. Tests:
   • Added widget tests verifying cells do not mount ContextMenu or Tooltip in their subtree.
   • Added interaction tests verifying hover over cell displays the tooltip overlay and mouse exit dismisses it.