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/compression.c

@@ -0,0 +1,46 @@
+#include "compression.h"
+#include "data.h"
+#include "log.h"
+#include "stdlib.h"
+#include "zstd.h"
+
+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;
+}