Anchor: Have the referring user site automatically selected in the OAuth site dropdown.

Currently, the OAuth site selector will load with the user's default site set at the top. A better user experience for those wanting to connect a new Anchor podcast to their site would be to have that site pre-selected instead.

See: https://github.com/Automattic/wp-calypso/issues/50815

Start by reading the OAuth site selector flow and the related issue #50815. Confirm how the referring user site is identified and where the selector's initial choice is set; done means the referring site is pre-selected while the existing default-site behavior remains available when no referring site applies.

Written by the indexing model from the issue text.