Based on #56 add ability to specify URI (either local file or HTTP endpoint) to use for GTFS archive. Add as negative feed id and otherwise UI could stay the same
Local file picker is most likely the most preferable route. (If user has a GTFS URL to use directly at that point you'd probably want to just submit to mobility-database-catalogs upstream, right?).
Filepicker in gio-x to use: https://pkg.go.dev/gioui.org/x/explorer#Explorer.ChooseFile
Example: https://github.com/gioui/gio-example/blob/main/explorer/main.go
New deps for alpine: xdg-desktop-portal-gtk
Branch:
custom-gtfs-nom