refactor: split monolithic modules and extract shared components

- Remove dead socket.c/socket.h (accumulated duplicate symbols)
- Extract compression.h/c (data_compress/decompress from data.c)
- Extract io.h/c (low-level I/O from protocol.c)
- Extract metadata.h/c (unified metadata wire format from file.c, chunk.c, utils.c)
- Split client.c into client_cli.c (CLI parsing) + client_send.c (send logic)
- Move receiver pipeline threads from server.c to multiprocessing.c
- Move to_disk/file_restore_metadata from utils.c to file.c/metadata.c
- Fix const qualifiers on to_disk and str_dup signatures
- Suppress chown unused-result warning

src/shared/metadata.h

@@ -0,0 +1,15 @@
+#ifndef METADATA_H
+#define METADATA_H
+
+#include "file.h"
+#include <sys/stat.h>
+
+#define FILE_METADATA_WIRE_SIZE (sizeof(mode_t) + sizeof(uid_t) + sizeof(gid_t) + sizeof(time_t) + sizeof(long))
+
+void metadata_to_buf(char **buf, FileMetadata *m);
+FileMetadata *metadata_from_buf(char **buf);
+void metadata_send(int file_descriptor, FileMetadata *m);
+FileMetadata *metadata_receive(int file_descriptor);
+void file_restore_metadata(const char *path, FileMetadata *metadata);
+
+#endif