Add per-unit visibility dropdown (Private / Public) directly on the
repo settings page next to each unit's enable checkbox. This replaces
the need to navigate to a separate Public Access settings page.
Supported units: Code, Wiki, Issues, Releases. Each gets a dropdown
that controls AnonymousAccessMode — when set to Public, the unit is
readable by anonymous visitors even on private repos.
Closes#238
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>