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.h

@@ -12,6 +12,7 @@ typedef struct Config {
   bool use_chunk_serialization;
   bool use_compression;
   bool use_single_send_per_file;
+  bool use_metadata;
   int compression_level;
   int num_connections;
 } Config;
@@ -19,8 +20,8 @@ typedef struct Config {
 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);
 void config_delete(Config *config);
 void config_send(int file_descriptor, Config *config);
 Config *config_receive(int file_descriptor);