clipboard: notify the audit at the first file of a list, not the last

wf_cliprdr_add_to_file_arrays() assigned first_file_index for every
regular file, so it ended up holding the last one, and HandleClipboardFiles
fired when the paste reached the last file rather than when it started. A
paste cancelled before its last file sent no audit record. The Unix owner
in serv_files.rs already takes the first regular file.

Keep the first assignment only. clear_file_array() resets the index to
(size_t)-1 before every list is built.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JAxkkiaGp2k8okvs4MpCUq

libs/clipboard/src/windows/wf_cliprdr.c

@@ -2561,7 +2561,8 @@ static BOOL wf_cliprdr_add_to_file_arrays(wfClipboard *clipboard, WCHAR *full_fi
 	}
 
 	if ((clipboard->fileDescriptor[clipboard->nFiles]->dwFileAttributes &
-		 FILE_ATTRIBUTE_DIRECTORY) == 0) {
+		 FILE_ATTRIBUTE_DIRECTORY) == 0 &&
+		clipboard->first_file_index == (size_t)-1) {
 		clipboard->first_file_index = clipboard->nFiles;
 	}