fix(diffviewer): wrap long lines through delta in side-by-side, mark clipped lines in unified (closes #99)

[Booyaka101]

Summary

Closes #99.

diffFile and diffDir invoke delta with --max-line-length=<width>, which is the truncation flag — not a wrap hint. Combined with delta's default --wrap-max-lines=2, any source line longer than roughly two viewport widths got silently dropped in side-by-side mode. In unified mode (where delta does not wrap at all) the long line then hit the post-processing truncation in the diffContentMsg handler, which used an empty tail ("") and clipped without any visible signal.

The reporter pointed at both spots in the original issue — both the delta args and the post-processing pass.

Fix

Two changes, both in pkg/ui/panes/diffviewer/diffviewer.go:

1. Both diffFile and diffDir — replace --max-line-length=<width> with --max-line-length=0 (disable truncation) and add --wrap-max-lines=unlimited (let delta wrap as many times as needed). Side-by-side mode now wraps long lines all the way through instead of stopping at the second wrap.

2. diffContentMsg handler — change the ansi.Truncate tail from "" to …. Unified mode (where delta does not wrap) at least surfaces a visible cut-off marker instead of vanishing the remainder.

Verification

Local repro with the canonical "very long line" diff at -w=80:

Mode	Pre-fix	Post-fix
Side-by-side	clipped at "what flags delta is invoked with" (≈2 wraps)	reaches "to see what happens here" (full content)
Unified	line silently cut at viewport width with no marker	line cut at viewport width with … tail

Verified the args delta receives by running them directly:

$ delta --paging=never -w=80 --max-line-length=0 --wrap-max-lines=unlimited --side-by-side < long.diff
…
│    │                                  │  2 │const Long = "this is a very long↵
│    │                                  │    │ line that should be wrapped or t↵
│    │                                  │    │runcated depending on what flags ↵
│    │                                  │    │delta is invoked with and we want↵
│    │                                  │    │ to see what happens here"
…

Existing pkg/ui/panes/diffviewer test suite passes:

ok  	github.com/dlvhdr/diffnav/pkg/ui/panes/diffviewer	0.361s

The pre-existing TestSearchResultsRenderWhenFileTreeIsHidden, TestBuildFullFileTree, and TestCollapseTree failures on main are unrelated lipgloss-styling drift and are not touched by this PR.

Notes on scope

• Unified-mode horizontal scrolling is the natural follow-up — delta does not wrap there regardless of the flags above, so the only way to actually read a clipped line right now is s to switch to side-by-side. Happy to send a follow-up that adds horizontal scroll to the viewport if that's wanted, but kept this PR focused on the bug as filed.
• The --wrap-max-lines=unlimited value is delta's documented sentinel ("a value of unlimited means a line will be wrapped as many times as required").

Credit to @fgrehm for the report and the precise line references in the issue.