Add Nexus Mods OAuth and authenticated downloads (#1850)

* feat: add Nexus Mods OAuth authentication

* fix: harden Nexus OAuth review findings

* fix: block Nexus imports until authentication completes

* test: isolate Nexus import authentication states

* test: cover Nexus import authentication gate

* fix: preserve resumable local import service work

* fix: preserve Nexus callbacks across lifecycle changes

* fix: serialize Nexus OAuth callback processing