fix(server): check kill_recv in accept() loop to prevent hang on stop

If server_stop() is called before any client connects, the accept()
loop would spin forever since kill_recv was never checked, causing
pthread_join() to block indefinitely.

Add kill_recv check at the top of the accept() polling loop so the
thread can exit cleanly when server_stop() is called during the
pre-connection phase.

src/server/server.c

@@ -436,8 +436,12 @@ _start_server_cb(void* userdata)
     int c = sizeof(struct sockaddr_in);
     int client_socket;
     errno = 0;
-    // DEBUG removed
     while ((client_socket = accept(listen_socket, (struct sockaddr *)&client_addr, (socklen_t*)&c)) == -1) {
+        if (kill_recv) {
+            log_println(STBBR_LOGINFO, "Accept loop interrupted by stop request");
+            _shutdown();
+            return NULL;
+        }
         if (errno != EAGAIN && errno != EWOULDBLOCK) {
             log_println(STBBR_LOGERROR, "Accept failed: %s", strerror(errno));
             return NULL;