refactor: standardize error reporting on the log module

- Add log_perror() helper (context + strerror(errno)) to the log module
- Replace all bare perror() calls with log_perror() so errors are routed
  through the unified logger (stderr sink + optional --log-file sink)
- Convert fprintf(stderr, "Error:/Warning: ...") in client code to
  log_message(); raw fprintf kept only for progress/stats output

src/server/server.c

@@ -129,7 +129,7 @@ void handler(int file_descriptor) {
     if (receiver_created)
       writer_created = thrd_create(&writer, write_thread, context) == thrd_success;
     if (!receiver_created || !writer_created) {
-      perror("Error creating Threads");
+      log_perror("Error creating Threads");
       if (receiver_created) {
         mtx_lock(&context->mutex);
         atomic_store(&context->cancelled, true);