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/client/client_send.c

@@ -171,10 +171,11 @@ int send_files_multithreaded(Config *config) {
     return 1;
   }
 
+  int sender_result;
   thrd_join(scanner, NULL);
   thrd_join(loader, NULL);
-  thrd_join(sender, NULL);
+  thrd_join(sender, &sender_result);
 
   pipeline_context_sender_destroy(context);
-  return 0;
+  return sender_result == thrd_success ? 0 : -1;
 }