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/projects_files.go

@@ -113,9 +113,11 @@ func prjFilesUploadCmd() *cobra.Command {
 	var replace, allowDuplicate bool
 	cmd := &cobra.Command{
 		Use:   "upload PROJECT_ID LOCAL_PATH [LOCAL_PATH...]",
-		Short: "Upload file(s) into the project's Documents folder (upsert by stem|ext)",
-		Long: `Default: replace an existing file with the same logical name (stem|ext), like cp overwrite.
-Pass --no-replace to fail when the name is taken; --allow-duplicate to always create a new file id.`,
+		Short: "Upload file(s) into the project's Documents folder (upsert by stem, legacy↔OOXML ext)",
+		Long: `Default: replace an existing file with the same logical name (stem, treating
+legacy .xls/.doc/.ppt and their OOXML .xlsx/.docx/.pptx as one file), like cp
+overwrite. Pass --no-replace to fail when the name is taken; --allow-duplicate
+to always create a new file id.`,
 		Args: cobra.MinimumNArgs(2),
 		RunE: func(cmd *cobra.Command, args []string) error {
 			c, err := newOO(cmd)