refactor: merge File/FileReceive, add optional metadata transfer with -M flag

- Remove FileReceive struct; use File everywhere with nullable FileMetadata*
- Remove struct stat from File; file size lives in Data->size (data_create_reserve)
- Add FileMetadata struct (mode, uid, gid, mtime) sent conditionally over wire
- Add -M / --preserve flag to client
- Restore permissions, ownership, timestamps on disk write
- Wire format uses per-file present flag for metadata (zero overhead when off)

src/shared/config.c

@@ -7,8 +7,8 @@
 Config *config_create(char *version, char *send_directory,
                       char *receive_directory, bool save_to_disk,
                       bool use_multithreading, bool use_chunk_serialization,
-                      bool use_compression, int compression_level,
-                      int num_connections) {
+                      bool use_compression, bool use_metadata,
+                      int compression_level, int num_connections) {
 
   Config *config = malloc(sizeof(Config));
   config->version = version;
@@ -18,6 +18,7 @@ Config *config_create(char *version, char *send_directory,
   config->use_multithreading = use_multithreading;
   config->use_chunk_serialization = use_chunk_serialization;
   config->use_compression = use_compression;
+  config->use_metadata = use_metadata;
   config->compression_level = compression_level;
   config->num_connections = num_connections;
   return config;
@@ -38,6 +39,7 @@ void config_send(int file_descriptor, Config *config) {
   send_int(file_descriptor, config->use_multithreading);
   send_int(file_descriptor, config->use_chunk_serialization);
   send_int(file_descriptor, config->use_compression);
+  send_int(file_descriptor, config->use_metadata);
   send_int(file_descriptor, config->compression_level);
   send_int(file_descriptor, config->num_connections);
   if (receive_status(file_descriptor) != STATUS_OK) {
@@ -55,6 +57,7 @@ Config *config_receive(int file_descriptor) {
   config->use_multithreading = receive_int(file_descriptor);
   config->use_chunk_serialization = receive_int(file_descriptor);
   config->use_compression = receive_int(file_descriptor);
+  config->use_metadata = receive_int(file_descriptor);
   config->compression_level = receive_int(file_descriptor);
   config->num_connections = receive_int(file_descriptor);
   send_status(file_descriptor, STATUS_OK);