feat: add unified diff view option

[markdorison]

Summary

Adds the ability to toggle between side-by-side and unified diff views, addressing #10 and completing the work started in #46.

• Add --side-by-side/-s and --unified/-u CLI flags
• Add ui.sideBySide config option (default: true)
• Add s keybinding to toggle view mode at runtime

Details

The implementation follows the existing iconStyle pattern: config sets the default, CLI flags override per-invocation, and keyboard shortcut toggles at runtime.

Priority order: CLI flag > Config > Default (side-by-side)

This addresses the PR #46 feedback by providing both flags, allowing users who prefer delta's convention to use --unified as their default via shell alias while preserving diffnav's existing behavior.

Usage

# Force unified view
git diff | diffnav --unified
git diff | diffnav -u

# Force side-by-side view
git diff | diffnav --side-by-side
git diff | diffnav -s

# Config file (~/.config/diffnav/config.yml)
ui:
  sideBySide: false  # Start in unified mode

# Toggle at runtime: press 's'

Test plan

• git diff | diffnav shows side-by-side for modified files (default)
• git diff | diffnav --unified shows unified view
• git diff | diffnav -u shorthand works
• Press s toggles view and re-renders current diff
• View mode persists when navigating between files
• New/deleted files always show unified (side-by-side makes no sense for them)
• Config sideBySide: false starts in unified mode
• CLI flag overrides config setting

Closes #10

[dlvhdr]

this is really good! thank you!
just had a minor nit

[dlvhdr]

Can we keep NewProgram to recieve only 2 params? Let's just merge the
sideBySide flag into the existing config and override its value if the user
specified the flag explicitly.

[markdorison]

Updated!