refactor: split file.c into file_send.c and file_receive.c

- file.c: File/FileMetadata lifecycle and local disk helpers (~110 lines)
- file_send.c: client-side send path (file_send_single_calls, file_send_sendfile)
- file_receive.c: server-side receive/save path (file_receive, receive_incremental_check,
  receive_manifest, file_save_to_disk)
- file_types.h holds shared struct definitions; file.h remains an umbrella header
  so existing includes are unaffected
Completes the transfer/protocol separation started in PR #212

src/shared/file_send.h

@@ -0,0 +1,14 @@
+#ifndef FILE_SEND_H
+#define FILE_SEND_H
+
+#include "file_types.h"
+#include <stdbool.h>
+
+/* Client-side file send path. */
+
+bool file_send_single_calls(File* file, int file_descriptor, bool use_metadata,
+                            int compression_level, bool send_path);
+bool file_send_sendfile(File* file, int file_descriptor, bool use_metadata, int compression_level,
+                        bool send_path);
+
+#endif