Data Editor: Add Bulk Fill and Undo/Redo Functionality

Enable Excel-style copy→select-range→paste-fill behavior in the data editor, and introduce undo/redo functionality for safer data manipulation.

## Summary
Partners are requesting the ability to perform multi-cell paste or "bulk fill" actions within the Data Editor, similar to Excel or Google Sheets. The current copy-paste functionality only supports one-to-one pasting, which limits productivity for repetitive data entry or adjustments across multiple cells. Additionally, the absence of undo/redo makes any bulk data operation risky and time-consuming to correct.

## Current Problem
In the current Data Editor, users cannot select multiple target cells (contiguous or non-contiguous) and paste a copied value into them all at once. This limitation slows workflows for partners managing large datasets or needing to apply the same value across multiple rows.
The lack of undo/redo further compounds this issue — users are hesitant to perform bulk edits or corrections because errors cannot be reverted.

## Examples/Scenarios
- A partner managing hundreds of rows needs to update the same "Status" or "Category" field across a selected range. Currently, they must paste into each cell individually.
- Users often want to fill non-adjacent cells (e.g., selected via Cmd/Ctrl + click) with a single copied value — a standard spreadsheet behavior that’s not supported.
- Accidental pastes or overwrites require manual correction because there’s no undo/redo safety net.

**Reference:** [Loom Video Example](https://www.loom.com/share/23a003cdbdf943be862482ff3d667b87)

## Why This Matters
Implementing bulk fill and undo/redo would:
- Dramatically improve data editing speed and reliability in the Data Editor.
- Reduce user frustration and perceived friction compared to traditional spreadsheet workflows.
- Build confidence among partners who use the Data Editor as a daily operational tool.
- Decrease the risk of irreversible data errors, supporting safer bulk operations.

## Suggested UX
- **Bulk Fill via Copy/Paste:**
- Allow users to copy a single value (or range) and paste it into multiple selected cells.
- Support both contiguous and non-contiguous range selection via Shift and Cmd/Ctrl clicks.
- Optionally prompt for confirmation when pasting into large ranges.

- **Undo/Redo:**
- Add visible Undo/Redo buttons to the Data Editor toolbar.
- Support standard keyboard shortcuts (`Cmd/Ctrl + Z` / `Cmd/Ctrl + Shift + Z`).
- Maintain a short action history buffer for safe rollback of recent changes.

Together, these features would align the Data Editor more closely with familiar spreadsheet paradigms while maintaining data integrity safeguards.