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.

files.go

@@ -237,7 +237,8 @@ func (c *Client) UploadProjectFile(ctx context.Context, projectID, localPath str
 	return decodeResponseFileEntry(raw)
 }
 
-// UploadProjectFileReplacing upserts by stem|ext in the project Documents folder.
+// UploadProjectFileReplacing upserts by stem and (server-converted) extension
+// in the project Documents folder.
 func (c *Client) UploadProjectFileReplacing(ctx context.Context, projectID, localPath string) (*FileEntry, []int, error) {
 	folderID, err := c.projectFolderID(ctx, projectID)
 	if err != nil {