fix(datagrid): redraw cells after Cmd+Z when editor view briefly disappears

[datlechin]

Summary

After editing a cell and pressing Cmd+Z, the yellow "modified" cell background and the modified-row highlight stayed on screen, even though pendingChanges and the toolbar Save button correctly cleared. The redraw call after undo was silently dropped.

Root cause: MainEditorContentView was wiring its @State dataTabDelegate into coordinator.dataTabDelegate (a weak ref) on .onAppear, and explicitly nilling that ref on .onDisappear. SwiftUI fires onDisappear on transient lifecycle events even when the @State strong owner persists, so the wire briefly went nil between the cell edit and the Cmd+Z dispatch. handleUndoResult then walked through dataTabDelegate?.tableViewCoordinator?.invalidateCachesForUndoRedo() and got an early-out via the nil chain.

The explicit nil is unnecessary. weak var clears itself when the @State strong owner is finally deallocated on permanent view destruction. Until then the wire stays valid through hide/show cycles. Same fix protects every other coordinator method that hits dataTabDelegate?.tableViewCoordinator?... (delete, paste, duplicate, load-more, undo-insert, etc.) which were all silently no-oping on the same code path.

Test plan

• Open any editable table tab, edit a single cell, press Cmd+Z — yellow background and row highlight clear immediately, toolbar Save stays disabled
• Edit a cell, switch to Structure tab and back, press Cmd+Z — same as above
• Insert a new row, edit a cell on it, press Cmd+Z twice — cell change reverts, then row insertion reverts, no stuck "inserted" stripe
• Delete rows, undo — rows reappear, no orphan deleted-row tint

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