Phase 1+2: bug fixes and dead code cleanup

Phase 1 — Bugs:
- receive_data: fix unsigned long long vs size_t mismatch
- chunk_deserialize: add NULL check in multiprocessing.c and server.c
- file_content_to_buffer: add missing fclose on error path
- send_files_multithreaded: propagate sender thread result
- server: add SIGPIPE handler
- to_disk: check fwrite return value
- scanner: use S_ISDIR instead of !S_ISREG
- scanner: free cur_path before early return

Phase 2 — Cleanup:
- Remove unused chunk_decompress function
- Remove unused array_list_clear function
- Remove unused num_connections config field
- Remove duplicate FILE_METADATA_WIRE_SIZE macro
- Update tests for removed APIs

src/shared/config.h

@@ -19,7 +19,6 @@ typedef struct Config {
   bool use_sendfile;
   bool use_metadata;
   int compression_level;
-  int num_connections;
   TransportType transport;
   char *ssh_destination;
 } Config;
@@ -28,7 +27,7 @@ Config *config_create(char *version, char *send_directory,
                       char *receive_directory, bool save_to_disk,
                       bool use_multithreading, bool use_chunk_serialization,
                        bool use_compression, bool use_metadata,
-                       int compression_level, int num_connections, bool use_sendfile);
+                       int compression_level, bool use_sendfile);
 void config_delete(Config *config);
 void config_send(int file_descriptor, Config *config);
 Config *config_receive(int file_descriptor);