Update MO2 to use OAuth 2.0 instead of API keys from Nexus Mods

[Pickysaurus]

Is your feature request related to a problem? Please describe.
The API key authentication method MO2 currently uses is slowly being deprecated in favour of an OAuth 2.0 system. While it doesn't impact anything directly today, over time, some features of Nexus Mods may stop working with API keys and require an OAuth session instead (this is due to OAuth being tied to user permissions more closely).

Describe the solution you'd like
Update the MO2 sign-in to Nexus Mods to use OAuth 2.0 instead of API keys.

A guide for how to do this is available here. I am available on Nexus Mods and Discord if anyone picking up this ticket needs some support (we'll also need to register you for a client ID when doing this).

This also unlocks potential future integrations, like being able to download/upload collections in the MO2 UI directly.

[LightningManGTS]

No idea how relevant this is, but today I am getting a 500 error at https://api.nexusmods.com/v1/users/validate when MO2 tries to use the api key to login.

here's the full error message: [2025-09-14 13:00:31.419 E] nexus: Error transferring https://api.nexusmods.com/v1/users/validate - server replied: (500)