feat(csv): find & replace panel with preview before apply

[salmonumbrella]

Problem

Find & Replace on a CSV file is currently limited to single-cell editing. CSV editors ship a Find & Replace panel that highlights matches live, scopes to selection / column, and previews the replacement set before commit. The preview step prevents the classic "replaced 4,000 cells, can't tell what changed, can't undo cleanly" failure mode.

Proposed solution

A Find & Replace panel that:

• Highlights matches in the grid as the user types, with a running match count
• Supports literal, regex, case-sensitive, and whole-cell match modes
• Scopes to All cells / Selected columns / Selected cells / Current column — driven by feat(datagrid): support column and cell-range selection #1446's selection
• Shows a preview list of every replacement (before / after / row / column) with a checkbox per match; user clicks Apply to commit only the checked replacements
• Registers as one undo entry for the entire apply
• Opens via ⌘⇧F alongside the existing ⌘F (Find)

Optional: Replace in column… as a one-click action from the column-header right-click menu, prefilled with the column scope.

Implementation notes

• The cell-iteration primitive lives in the CSV datasource; the panel itself can reuse the existing filter-panel UI shell.
• feat(datagrid): support column and cell-range selection #1446's Set<CellIndex> is the natural representation for the preview's "matches still pending application" state.