Get fork branches

Returns branches from the authenticated user's fork of FPP (github.com/<user>/fpp) if that fork exists. Probed via GitHub API using saved gitHubUser/gitHubPAT; returns hasFork false when no credentials, no fork, or GitHub unreachable. Falls back to local git remote <user> when offline. Used by Developer settings to offer a fork-branch switcher defaulting to Use Official Branches.