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/data.c

@@ -1,8 +1,6 @@
-
 #include "data.h"
 #include "log.h"
 #include "stdlib.h"
-#include "zstd.h"
 
 Data *data_create_empty(size_t data_size) {
   void *data = malloc(data_size);
@@ -40,44 +38,3 @@ void data_destroy(Data *data) {
   free(data->data);
   free(data);
 }
-
-Data *data_compress(Data *data_to_compress, int compression_level) {
-  log_message(LOG_LEVEL_DEBUG, "Starting to compress data");
-  Data *compressed_data =
-      data_create_empty(ZSTD_compressBound(data_to_compress->size));
-
-  compressed_data->size = ZSTD_compress(
-      compressed_data->data, compressed_data->size, data_to_compress->data,
-      data_to_compress->size, compression_level);
-  if (ZSTD_isError(compressed_data->size)) {
-    log_message(LOG_LEVEL_ERROR, "Compression failed: %s",
-                ZSTD_getErrorName(compressed_data->size));
-    exit(EXIT_FAILURE);
-  }
-
-  log_message(LOG_LEVEL_DEBUG, "Data succesfully compressed from %zu to %zu",
-              data_to_compress->size, compressed_data->size);
-  return compressed_data;
-}
-
-Data *data_decompress(Data *compressed_data) {
-  log_message(LOG_LEVEL_DEBUG, "Start to decompress data");
-  Data *uncompressed_data = data_create_empty(
-      ZSTD_getFrameContentSize(compressed_data->data, compressed_data->size));
-  if (ZSTD_isError(uncompressed_data->size)) {
-    log_message(LOG_LEVEL_ERROR, "Decompression failed: %s",
-                ZSTD_getErrorName(uncompressed_data->size));
-    exit(EXIT_FAILURE);
-  }
-
-  uncompressed_data->size =
-      ZSTD_decompress(uncompressed_data->data, uncompressed_data->size,
-                      compressed_data->data, compressed_data->size);
-  if (ZSTD_isError(uncompressed_data->size)) {
-    log_message(LOG_LEVEL_ERROR, "Decompression failed: %s",
-                ZSTD_getErrorName(uncompressed_data->size));
-    exit(EXIT_FAILURE);
-  }
-  log_message(LOG_LEVEL_DEBUG, "Decompressed data successfully");
-  return uncompressed_data;
-}