refactor(files): single file client facade + CLI/TUI migration (#38)

- FileClient composes FileStore/Searcher backends; Read()/Write()/Search()
  select REST/DAV/PG/ES with transient read fallback. Client.Files() returns
  it and *FileClient implements FileStore, so existing callers keep working.
- Entry gains backend-native Updated + folder FilesCount/FoldersCount so
  dav ls output round-trips.
- cmd/oo dav/projects files and cmd/office/fetch download/preview/delete go
  through FileStore; oo search goes through the facade.
- Mark transport methods that FileStore now abstracts as deprecated.

cmd/office/fetch/list.go

@@ -17,6 +17,18 @@ const MailListPageSize = 25
 // Loader fetches list items for a menu subject using the OnlyOffice client.
 type Loader struct {
 	Client *onlyoffice.Client
+
+	// Files is the backend-agnostic file store used for file download, preview
+	// and delete. When nil it falls back to Client.FileStore(ProviderREST).
+	Files onlyoffice.FileStore
+}
+
+// fileStore returns the configured file store, defaulting to REST.
+func (l *Loader) fileStore() onlyoffice.FileStore {
+	if l.Files != nil {
+		return l.Files
+	}
+	return l.Client.FileStore(onlyoffice.ProviderREST)
 }
 
 // List returns items for the given list spec (nav leaf).
@@ -171,11 +183,7 @@ func (l *Loader) executeDelete(ctx context.Context, item model.Item) (string, er
 		}
 		return fmt.Sprintf("Deleted message %s", item.Title), nil
 	case model.KindFile:
-		id, err := strconv.Atoi(item.ID)
-		if err != nil {
-			return "", err
-		}
-		if err := l.Client.DeleteFiles(ctx, []int{id}); err != nil {
+		if err := l.fileStore().Delete(ctx, []string{item.ID}); err != nil {
 			return "", err
 		}
 		return fmt.Sprintf("Deleted file %s", item.Title), nil
@@ -199,7 +207,7 @@ func (l *Loader) executeDownload(ctx context.Context, item model.Item, destPath
 		return "", err
 	}
 	defer f.Close()
-	if _, err := l.Client.DownloadFile(ctx, item.ID, f); err != nil {
+	if _, err := l.fileStore().Download(ctx, item.ID, f); err != nil {
 		return "", err
 	}
 	return fmt.Sprintf("Downloaded to %s", destPath), nil