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/utils.h

@@ -1,12 +1,8 @@
 #ifndef UTILS_H
 #define UTILS_H
 
-#include "file.h"
-
 void mkdir_r(char *path);
-char *str_dup(char *string);
-void to_disk(char *path, void *data, unsigned long long data_size);
-void file_restore_metadata(const char *path, FileMetadata *metadata);
+char *str_dup(const char *string);
 char *path_cat(char *path1, char *path2);
 
 #endif