[Bug] EE becomes unresponsive if you cancel master server registration

Issue

If you attempt to register an EE server with the master server, then cancel to return to the ServerCreationScreen, EE becomes unresponsive until it gets a response from the master server.

Steps to reproduce

The master server is timing out right now, making this easily reproducible now but potentially not when the master server is working.

1. Launch EE as a server registered with the master server, either from the UI or with Preferences File options like headless_internet=1.
2. While waiting for the master server to respond, either cancel in the UI or attempt to SIGINT/ctrl-c to quit the headless server.

Expected behavior

EE kills the request to the master server and is immediately responsive in the UI or exits as a headless server.

Observed behavior

EE is unresponsive until it gets a response from the master server, including 504 timeouts.

Headless backtrace if killed while stalled:

Thread 1 "EmptyEpsilon" received signal SIGABRT, Aborted.
Downloading 1.79 K source file /usr/src/debug/glibc-2.41-11.fc42.x86_64/nptl/../sysdeps/unix/sysv/linux/x86_64/syscall_cancel.S
__syscall_cancel_arch () at ../sysdeps/unix/sysv/linux/x86_64/syscall_cancel.S:56                                          
56              ret
(gdb) bt
#0  __syscall_cancel_arch () at ../sysdeps/unix/sysv/linux/x86_64/syscall_cancel.S:56
#1  0x00007ffff767c75c in __internal_syscall_cancel (a1=<optimized out>, a2=<optimized out>, a3=<optimized out>, 
    a4=<optimized out>, a5=a5@entry=0, a6=a6@entry=4294967295, nr=202) at cancellation.c:49
#2  0x00007ffff767cdcc in __futex_abstimed_wait_common64 (private=128, futex_word=0x7fffabfff990, 
    expected=<optimized out>, op=<optimized out>, abstime=0x0, cancel=true) at futex-internal.c:57
#3  __futex_abstimed_wait_common (futex_word=futex_word@entry=0x7fffabfff990, expected=<optimized out>, 
    clockid=clockid@entry=0, abstime=abstime@entry=0x0, private=private@entry=128, cancel=cancel@entry=true)
    at futex-internal.c:87
#4  0x00007ffff767ce2f in __GI___futex_abstimed_wait_cancelable64 (futex_word=futex_word@entry=0x7fffabfff990, 
    expected=<optimized out>, clockid=clockid@entry=0, abstime=abstime@entry=0x0, private=private@entry=128)
    at futex-internal.c:139
#5  0x00007ffff7681c8c in __pthread_clockjoin_ex (threadid=140736079066816, thread_return=0x0, clockid=0, abstime=0x0, 
    block=<optimized out>) at pthread_join_common.c:108
#6  0x00007ffff784e45b in __gthread_join (__threadid=<optimized out>, __value_ptr=0x0)
    at /usr/src/debug/gcc-15.2.1-1.fc42.x86_64/obj-x86_64-redhat-linux/x86_64-redhat-linux/libstdc++-v3/include/x86_64-redhat-linux/bits/gthr-default.h:715
#7  std::thread::join (this=0x2781950) at ../../../../../libstdc++-v3/src/c++11/thread.cc:134
#8  0x00000000004996cc in GameServer::~GameServer ()
    at /home/oznogon/git/emptyepsilon-compile/SeriousProton/src/multiplayer_server.cpp:73
#9  0x00000000005264e0 in EpsilonServer::~EpsilonServer ()
    at /home/oznogon/git/emptyepsilon-compile/EmptyEpsilon/src/epsilonServer.h:10
#10 EpsilonServer::~EpsilonServer () at /home/oznogon/git/emptyepsilon-compile/EmptyEpsilon/src/epsilonServer.h:10
#11 0x000000000045e9e7 in P<GameServer>::release () at /home/oznogon/git/emptyepsilon-compile/SeriousProton/src/P.h:155
#12 P<GameServer>::check_release () at /home/oznogon/git/emptyepsilon-compile/SeriousProton/src/P.h:146
#13 P<GameServer>::operator bool () at /home/oznogon/git/emptyepsilon-compile/SeriousProton/src/P.h:123
#14 Engine::runMainLoop () at /home/oznogon/git/emptyepsilon-compile/SeriousProton/src/engine.cpp:199
#15 0x00000000004460bb in main () at /home/oznogon/git/emptyepsilon-compile/EmptyEpsilon/src/main.cpp:427

Read SeriousProton/src/multiplayer_server.cpp, especially GameServer::~GameServer, then follow Engine::runMainLoop in SeriousProton/src/engine.cpp and the registration path described in the issue. Reproduce with a timing-out master server; done means cancel or exit returns immediately without waiting for the master-server request or blocking during cleanup.