[MekDrop]

Summary

• Extends frontend/src/services/github.js with getUser, listRepos, getBranches, getTree, and a shared apiFetch helper; keeps fetchCurrentUser as a backwards-compatible alias
• Extends frontend/src/services/gitlab.js with getUser, listProjects, getBranches, getTree, and a shared glFetch helper; existing PKCE functions are untouched

Closes #9

Test plan

• npm -w frontend run lint passes with no errors
• Verify fetchCurrentUser still works (auth store compatibility)
• Manually verify GitHub API calls return expected shapes
• Manually verify GitLab API calls return expected shapes with custom host