Fixes#10.
Five bugs that caused the server to crash or become unresponsive:
1. Signal handler deadlock (main.c)
signal_handler called room_destroy (pthread_rwlock + free) and printf —
neither is async-signal-safe. If SIGTERM arrived while any thread held
g_room->lock, the process deadlocked permanently.
Fix: handler now only writes a message via write(2) and calls _exit(0).
Also remove close(g_listen_fd) which was closing stdin (fd 0), since
ssh_server_init returns 0 on success, not a real file descriptor.
2. NULL dereference in room_broadcast when room is empty (chat_room.c)
calloc(0, n) may return NULL per POSIX; memcpy on NULL is undefined.
Also: no NULL check after calloc for the OOM case.
Fix: early return if count == 0; check calloc return value.
3. Stack buffer overflow in tui_render_screen (tui.c)
char buffer[8192] overflows with tall terminals: 197 visible lines *
~1031 bytes/message ≈ 203 KiB. Title padding loop also lacked a
bounds check (buffer[pos++] = ' ' with no guard).
Fix: switch to malloc(65536) with buf_size used consistently.
Add bounds check to the title padding loop.
4. sleep() inside libssh auth callback (ssh_server.c)
auth_password is called from ssh_event_dopoll in the main thread.
sleep(2) there blocks the entire accept loop — one attacker with
repeated wrong passwords stalls all incoming connections.
IP blocking via record_auth_failure already handles brute force.
Fix: remove sleep(2) from auth_password.
5. Spurious sleep() calls in the main accept loop (ssh_server.c)
sleep(1/2) after rejecting rate-limited or over-limit connections
delays accepting the next legitimate connection for no benefit.
Fix: remove all sleep() from the accept loop error paths.
