Fix Velocity perform.login not reaching target server on cross-server switch

[lokspel]

Problem: When switching Velocity servers (e.g. survival → lobby), players get prompted to log in again. perform.login never reaches the new server because event.getPlayer().getCurrentServer() returns an empty Optional or the previous server (survival) during ServerConnectedEvent.

Fix:

1. Send perform.login via event.getServer() (the RegisteredServer the player just joined) instead of event.getPlayer().getCurrentServer() — always correct, no race condition.
2. Fix infinite retry loop in scheduleRetry(): increment attempt counter before checking getCurrentServer(), so it progresses even when the server connection isn't available yet.

[lokspel]

Fixes probably #3076