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/client/client_send.h

@@ -0,0 +1,15 @@
+#ifndef CLIENT_SEND_H
+#define CLIENT_SEND_H
+
+#include "chunk.h"
+#include "config.h"
+#include "transport_tcp.h"
+
+extern char *server_host;
+extern int server_port;
+
+int send_chunk(Client *client, Chunk *chunk, Config *config);
+int send_files(Config *config);
+int send_files_multithreaded(Config *config);
+
+#endif