Keep players logged in across server switches when the pre-join dialog is enabled

[sgdc3]

Problem

With a proxy + autoLogin: true, an already-authenticated player switching back to an auth server gets re-prompted to log in on Paper/Folia backends that have registration.dialog.preJoin.enable: true.

The pre-join dialog is shown during the connection configuration phase, which on a switch fires before the proxy sends perform.login (sent only from ServerConnectedEvent, after configuration completes). The dialog blocks that phase, so the auto-login can't arrive in time and the dialog always wins. Same symptom as #3076 (its perform.login routing part was already fixed in 63dccbf).

Fix

Send the auto-login during the configuration phase and let the backend act on it:

• Velocity — on PlayerEnteredConfigurationEvent, send the HMAC-signed perform.login to the auth server before the backend shows the dialog (ServerConnectedEvent path kept as backstop).
• Paper/Folia — new PaperProxyAutoLoginListener handles the configuration-phase PluginMessageListener overload; on a valid perform.login it queues the proxy session and dismisses the dialog (approvePreJoinForceLogin). BungeeReceiver.handleConfigPhasePerformLogin reuses the existing verifyHmac, and PaperDialogFlowListener re-checks the queue after registering the dialog future to close the residual race.

Not yet verified on a live Paper + Velocity network.

Refs #3076