feat: Improve diff viewer readability by highlighting header lines

[shatrughantwt]

Description

Fixes #36

This PR improves the readability of the TUI diff viewer by visually highlighting diff header lines.

What Changed

• Added a lightweight styleDiffContent() preprocessing step.
• Detects and styles lines starting with:
  • diff --git
  • index
  • ---
  • +++
  • @@
• Applies bold BrightMagenta styling for clearer visual distinction.
• Integrates styling before passing content to the viewport.

Why

Previously, diff headers blended with metadata and content lines, making multi-file diffs harder to scan.

This change improves visual hierarchy and makes it easier to quickly identify:

• File boundaries
• Hunk markers
• Metadata vs. content changes

[bakayu]

@shatrughantwt since this involves visual changes to the app, can you share screenshots/video recordings of how it is working on your end so that I can confirm that it is the same same and intended behavior as mine as I test it on my machine?
Thank you.

[shatrughantwt]

@shatrughantwt since this involves visual changes to the app, can you share screenshots/video recordings of how it is working on your end so that I can confirm that it is the same same and intended behavior as mine as I test it on my machine? Thank you.

• Multiple file diffs
• Large diffs (~150+ lines)
• Commit diffs
• Stash diffs