fix(diff): preserve tab indentation in diff view

[baggiiiie]

Bug:
Diff view dropped indentation on lines containing tabs, while preview mode rendered the same diff correctly.

Root cause:
The diff pane rendered lines directly with Ultraviolet, which does not expand tab characters. It also computed wrap and horizontal-scroll widths from the unexpanded text, so tabs were wrong for both drawing and layout.

Fix:
Expand tabs when diff content is loaded, before storing lines and computing widths. Use the same Lip Gloss tab expansion behavior as preview so rendering, wrapping, and horizontal scrolling all stay consistent. Add regression tests for rendering, wrapping, and horizontal scrolling with tab-indented lines.

Closes #604

issue before fix:

Preview works fine:

Diff view is breaking:

[idursun]

Hey thanks!

There's a comment I think we should address.

[baggiiiie]

@idursun i pushed another commit in order to deal with tabs not starting at column 0

the solution is getting longer, but it matches the default output from jj diff now

[idursun]

Hey thanks!

There are one comment about multi line treatment. One additional comment about potential deadcode.

[idursun]

We are already splitting by line before calling this function but then doing the same thing here. I'd expect ExpandTabs to deal with a single line and leave the splitting to the caller.

[baggiiiie]

good call, though preview doesn't originally split lines

jjui/internal/ui/preview/preview.go

Lines 163 to 168 in e7be3e0

	func (m *Model) SetContent(content string) {
	content = strings.ReplaceAll(content, "\r", "")
	m.reset()
	m.content = content
	m.view.SetContent(content)
	}

but diff does:

jjui/internal/ui/diff/diff.go

Lines 183 to 188 in e7be3e0

	content = "(empty)"
	}
	lines := strings.Split(content, "\n")
	maxWidth := 0
	for _, line := range lines {

[baggiiiie]

@idursun thanks for the detailed review!

i added line splitting for preview to keep the expandTab function a bit more lightweight.
as #575 "drop viewport and use render package instead", there's some discrepency between how these two models deal with rendering.

maybe we do decide to port preview to the same custom renderer? it seems nice to also have wrapping and width aware layout for preview.

for now, im keeping this PR simple and didn't touch that logic

[idursun]

Thanks!