Add line number gutter toggle (L key) #20
Description
It would be useful to see line numbers in the diff view, similar to how IntelliJ IDEA shows side-by-side old/new line numbers in the gutter.
Why:
- Mental map of the file — line numbers help build a spatial sense of where changes sit in the file, making it easier to orient and find them in an editor
- Code review context — when discussing a diff with teammates, you can reference exact line numbers ("check line 142") instead of describing the location
- Matching with editor — quickly jump to a specific line in your editor after spotting something in the diff
- Matching with CI/linter output — linter warnings and test failures reference line numbers; seeing them in the diff helps locate the issue without mental math from hunk headers
Proposed behavior:
- L key toggles a line number gutter on/off (runtime only, no CLI flag)
- Two columns before the existing +/- prefix: old line number (left), new line number (right)
- Context lines show both numbers, added lines show only new, removed lines show only old
- Numbers are right-aligned with dynamic column width based on the file's max line number
Example:
▶ 25 32 + content here
▶ 40 - removed line
▶ 40 + added line
▶ 41 42 context line
- Works in normal, wrapped, and collapsed modes
- '#' icon in the status bar indicates active state
- Numbers styled with the existing muted color (color-muted)
Happy to submit a PR if this sounds reasonable.