fix: restore individual cell values on undo for multi-cell paste

[ghiscoding]

fixes #917

vibe coded a fix with Copilot

Description

Fixes issue #917 where undoing a multi-cell paste operation in CellExternalCopyManager incorrectly restores all cells to the same value instead of their individual previous values.

Root Cause

When pasting a single cell value to multiple cells (oneCellToMultiple), the undo function was incorrectly retrieving the old value. It used oldValues[0][0] for all cells instead of oldValues[y][x - xOffset], causing all cells to be restored to the first cell's previous value.

Reproduction Steps

1. Open example-excel-compatible-spreadsheet.html
2. Copy a single cell value
3. Select a range of multiple cells
4. Paste the single value (applies to all selected cells as expected)
5. Press Ctrl+Z to undo
6. Bug: All cells are incorrectly restored to the same value

Solution

• Removed the oneCellToMultiple condition from the undo function
• Added proper xOffset tracking to handle hidden columns correctly (consistent with the execute function)
• Each cell now restores from its own stored old value: oldValues[y][x - xOffset]

Testing

1. Copy a single cell with value "A"
2. Select a range of 4 cells (e.g., containing "B", "C", "D", "E")
3. Paste (all 4 cells now contain "A")
4. Undo (all 4 cells should be restored to their original values "B", "C", "D", "E")
5. Verify each cell is restored correctly

Impact

• Fixes undo behavior for multi-cell paste operations
• No breaking changes
• Properly handles hidden columns in the restore process

[ghiscoding]

validated the fix in Slickgrid-Universal in the UI and it works and the fix is valid