fix: text-spacing in diff overlay and help menu

[delianides]

Summary

Fixes two TUI spacing bugs:

• Diff overlay line numbers collided past 4 digits. The gutter used two fixed 4-wide columns ({:>4}{:>4} ) separated only by alignment padding, so a 5+ digit line number overflowed the field and the old/new numbers ran together as "one long digit." Added a gutter_width pre-scan that sizes both columns to the file's largest line number, and a format_gutter helper that always keeps a single-space separator. Narrow files stay compact; large files never collide.
• Help menu keys misaligned. Keys were padded to a fixed 14 chars, but several (Ctrl+u / Ctrl+d, gg / G / Ctrl+u/d/f/b) exceed that and overflowed, breaking the description column. Now computes a global key-column width from the widest key by terminal display width (via fit::display_width, so arrow glyphs are measured correctly) plus a 2-column gap.

Pure spacing — no new glyphs, colors, layout changes, or config options.

Test Plan

• cargo test — 356 pass (10 new: 5 format_gutter, 3 gutter_width, 2 help alignment)
• cargo clippy --all-targets -- -D warnings — clean
• cargo build --release — succeeds
• Visual check: open a diff touching line numbers ≥ 10000 — old/new numbers visibly separated and columns aligned
• Visual check: press ? — all descriptions start at the same column, including the longest key rows

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