fix a segfault on signals received after main returns

The waybar process does not exit instantaneously.
Signals may be recevied after main has started freeing resources.

When a worker thread is in `fgets` this time window can last forever.
An easy way to duplicate the crash is pressing ^C twice with a Hyprland module.

    Thread 1 "waybar" received signal SIGSEGV, Segmentation fault.
    spdlog::sinks::sink::should_log (this=0x5f620b542ca5,
        msg_level=spdlog::level::info)
        at /usr/src/debug/spdlog/spdlog-1.14.1/include/spdlog/sinks/sink-inl.h:13
    13	  return msg_level >= level_.load(std::memory_order_relaxed);
    (gdb) p $_siginfo._sifields._sigfault.si_addr
    $1 = (void *) 0x5f620b542cad

src/main.cpp

@@ -107,6 +107,10 @@ int main(int argc, char* argv[]) {
       ret = client->main(argc, argv);
     } while (reload);
 
+    std::signal(SIGUSR1, SIG_IGN);
+    std::signal(SIGUSR2, SIG_IGN);
+    std::signal(SIGINT,  SIG_IGN);
+
     delete client;
     return ret;
   } catch (const std::exception& e) {