## Summary
- Enable VSCode-like side-by-side file comparison in FileDiffView using
SplittableEditor
- Leverages existing SplittableEditor infrastructure that ProjectDiff
already uses
- Controlled by the user's `diff_view_style` setting (split/unified)

## Changes
- `crates/git_ui/src/file_diff_view.rs`: Refactored to use
SplittableEditor instead of plain Editor (+66/-30 lines)

## Test plan
- [ ] Open a file with git changes and verify the diff view renders
correctly
- [ ] Toggle between split and unified diff view styles via settings
- [ ] Verify side-by-side comparison shows old/new content correctly

Release Notes:

- Enabled side by side diffs in the file diff view

Co-authored-by: Lukas Wirth <me@lukaswirth.dev>