Add multi-platform git credential management

- Add git-credential-oauth package for OAuth authentication
- Configure OAuth helpers for GitHub, GitLab, BitBucket
- Add token-based authentication for Overleaf
- Remove disabled credential helper configuration
- Support secure browser-based OAuth flows and token storage

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

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

flake.nix

@@ -68,6 +68,7 @@
         nerd-fonts.jetbrains-mono
         gitui
         ncdu
+        git-credential-oauth
       ];
 
       fonts.fontconfig.enable = true;