refactor: config_create(), main(), send_files() - reduce duplication and complexity

- #150: Replace 11-parameter config_create() with config_create() that
  initializes to sensible defaults; callers set fields directly
- #149: Extract validate_config() from main(); reduce main() from 325 to
  281 lines by extracting validation logic into separate function
- #151: Extract run_dry_run(), connect_to_server(), send_manifest(), and
  print_progress() shared helpers from send_files()/send_files_multithreaded()
  to eliminate code duplication

tests/test_file.c

@@ -154,8 +154,9 @@ static void test_file_send_receive() {
   memcpy(file->data->data, content, len);
   file->data->size = len;
 
-  Config* cfg = config_create(str_dup(PROTOCOL_VERSION), str_dup("/tmp"), str_dup("/tmp"), false,
-                              false, false, false, false, 0, false, 0);
+  Config* cfg = config_create();
+  cfg->send_directory = str_dup("/tmp");
+  cfg->receive_root_directory = str_dup("/tmp");
 
   int p[2];
   EXPECT_EQ_INT(pipe(p), 0);