fix(dav): upsert upload matches server-converted ext (xls→xlsx) (#82)

OnlyOffice converts legacy binary Office uploads (.xls/.doc/.ppt) into
OOXML (.xlsx/.docx/.pptx) on the server. UploadToFolderReplacing matched
by the exact stem|ext via FindFilesByDedupKey, so a repeated
`oo dav upload FOLDER f.xls --replace` never found the stored f.xlsx and
appended a second file (live: ids 3799+3887, 3800+3888).

- EquivalentUploadExt / FindFilesByStemExt: match by stem with a
  legacy↔OOXML extension equivalence, so .xls finds the saved .xlsx.
- planUploadReplacement: pick the surviving file and the redundant
  duplicate ids for a replacing upload.
- UploadToFolderReplacing updates the existing file in place (UpdateFile,
  stable id, no delete window), removes extra duplicates, and falls back
  to conversion-aware delete + upload when the portal rejects the update.
- AssertNoFileConflict (--no-replace) uses the same conversion-aware
  matching so a .xls upload conflicts with an existing .xlsx.
- Offline tests cover the matcher, the plan and the repeated-.xls
  regression; pdf/xlsx behaviour unchanged.

cmd/oo/dav.go

@@ -253,7 +253,7 @@ func davUploadCmd() *cobra.Command {
 			return nil
 		},
 	}
-	cmd.Flags().BoolVar(&replace, "replace", true, "replace same stem|ext before upload (default); false = fail if name taken")
+	cmd.Flags().BoolVar(&replace, "replace", true, "replace the same logical file (stem, legacy↔OOXML ext) before upload (default); false = fail if name taken")
 	return cmd
 }
 
@@ -271,9 +271,10 @@ type uploader interface {
 	UploadToFolder(ctx context.Context, folderID, localPath string) (*onlyoffice.FileEntry, error)
 }
 
-// uploadLocal uploads each local file into folderID. With replace a same
-// stem|ext file is removed first (the UploadToFolderReplacing upsert); without
-// replace a name clash fails with ErrFileExists before anything is sent.
+// uploadLocal uploads each local file into folderID. With replace the existing
+// same logical file is overwritten in place (the UploadToFolderReplacing
+// upsert, which matches the server-converted extension too); without replace a
+// name clash fails with ErrFileExists before anything is sent.
 func uploadLocal(ctx context.Context, up uploader, folderID string, paths []string, replace bool) ([]uploadedFile, error) {
 	results := make([]uploadedFile, 0, len(paths))
 	for _, path := range paths {