feat(oauth): start the provider redirect through a port #128

Context

OAuthProviderButtons starts the provider redirect with:

window.location.assign(data.authorizationUrl);

There is no window.location in React Native. A native client has to open the
authorization URL in an in app browser session and receive the result back through the
app's URL scheme.

Scope

• Put the redirect behind a port, with a window.location implementation on web and an
  in app browser session (expo-web-browser) on native.
• Handle the callback through the app scheme rather than a route in a DOM router.

Notes

The API side already supports this. resolveOAuthRedirectUri matches a requested
redirect URI exactly against the provider allowlist before falling back to origin
comparison, so a custom scheme such as myapp://oauth/callback is accepted today with no
API change, as long as it is registered on the provider.