## Data table: Add left inset for scrollbar alignment with pinned
columns

When pinned columns are enabled in the data table, the horizontal
scrollbar previously extended across the full table width, including
behind the pinned columns. This change adds a left inset to the
scrollbar so it only spans the scrollable area, properly aligning with
the content layout.

## Demo (look at bottom scrollbar)
| Before | After|
| --- | --- |
| <img width="1614" height="482" alt="image_2026-05-20_12-29-22"
src="https://github.com/user-attachments/assets/87050173-0f3a-46dd-a7e1-10f769c7b5b8"
/> | <img width="1526" height="420" alt="image_2026-05-20_12-32-48"
src="https://github.com/user-attachments/assets/5041069f-2289-449b-8d30-8bcb3d0890eb"
/> |
### Changes

- Add `pinned_width()` and `scrollable_width()` helper methods to
`ResizableColumnsState` to compute column widths
- Refactor scrollbar rendering to use absolute positioning with inset
when pinned columns are present
- Scrollbar now correctly positions itself to span only the scrollable
(non-pinned) column area

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Improved horizontal scrollbar alignment when pinned columns are active

- N/A or Added/Fixed/Improved ...

---------

Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>