The Connect button on /settings/authentication pointed at the
auth.{provider}.redirect routes that live behind `guest` middleware,
so authenticated users were bounced to /app/home before reaching
Socialite. The OAuth callback also needed to handle two flows
(signup/login vs link to current user) but had no branch for the
second case — meaning a different-email GitHub account would have
been registered as a new user, logging the original session out.
Splits the flows by intent:
- New `app.authentication.connect-provider` route in the auth group,
handled by the settings controller (where it sits next to
disconnect-provider). Replaces the OAuth signup link as the
Connect button's target.
- Auth callbacks moved out of the guest group (still one URL per
provider, since OAuth apps only register one) and gain a single
Auth::check() branch that calls connectToCurrentUser().
- connectToCurrentUser() rejects if the provider id already belongs
to a different user; otherwise sets it on the current user and
redirects back to settings with a flash message.
