Cleans up table cell/row styling and reworks column header filter/sort
buttons so they stay out of the way until needed, with a gradient fade
instead of a hard clip when the header label is truncated.

### Summary

Improved CSV preview column headers to only show filter/sort actions on
hover, with a gradient fade for truncated labels

### Demo (todo)

| Before | After |
| --- | --- |
| <img width="771" height="626" alt="image"
src="https://github.com/user-attachments/assets/76dcf7b9-ed4a-444f-91f7-57f2acd16934"
/> | <img width="774" height="603" alt="image"
src="https://github.com/user-attachments/assets/aeda6693-ff37-41e6-a849-780874e23286"
/> |

### Testing

- Hover over a data column header: filter/sort buttons fade in, label
truncates behind a gradient rather than a hard edge
- Apply a filter or sort on a column: its buttons stay visible without
hovering
- Toggle row identifier / cell vertical alignment setting between top
and center: alignment applies consistently to both row identifiers and
cells
- Confirm table renders without the previous striping (only bottom
borders)

Release Notes:

-  N/A