Add gitui for beautiful CLI git graph visualization

- Add gitui package to home.packages for enhanced git repository browsing
- Provides interactive TUI with beautiful branching visualization
- Supports diff viewing, staging, and vim-like navigation

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

flake.nix

@@ -42,6 +42,7 @@
         claude-code.packages.aarch64-darwin.claude-code
         nerd-fonts.fira-code
         nerd-fonts.jetbrains-mono
+        gitui
       ];
 
       fonts.fontconfig.enable = true;