Auto sign in to coder

[juhannc]

When signing in to padws (self-hosted) via Keycloak, there should be an option to auto-sign in to coder without logging in again instead of clicking the sign-in button again.
I know it's a super small impact (literally one click less) but I think it could improve the flow quite a bit.

[atyrode]

It should actually work this way!
It happens in the file AuthGate.tsx for now, but there's a leftover hardcoded value which points to our instance of Coder at https://coder.pad.ws instead of reading from the .env

[atyrode]

Fixed by #51 / #52