Extract all feature unit settings (Code, Wiki, Issues, Projects,
Releases, Packages, Pull Requests) from options.tmpl into a
separate advanced.tmpl with its own route at /settings/advanced.
Options page now only contains: basic repo settings, avatar,
mirror config, signing settings, and danger zone.
Navbar updated: Advanced Settings link points to /settings/advanced.
Form posts still go through the existing SettingsPost handler.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>