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/preview.go

@@ -5,7 +5,6 @@ import (
 	"context"
 	"fmt"
 
-	onlyoffice "github.com/eslider/go-onlyoffice"
 	"github.com/eslider/go-onlyoffice/cmd/office/model"
 	"github.com/eslider/go-onlyoffice/cmd/office/preview"
 )
@@ -30,13 +29,11 @@ func (l *Loader) filePreviewMarkdown(ctx context.Context, item model.Item) (stri
 		return "", fmt.Errorf("file id missing")
 	}
 	name := item.Title
-	if meta, err := l.Client.GetFile(ctx, item.ID); err == nil && meta != nil {
-		if t := onlyoffice.FileEntryTitle(meta); t != "" {
-			name = t
-		}
+	if e, err := l.fileStore().Stat(ctx, item.ID); err == nil && e.Title != "" {
+		name = e.Title
 	}
 	var buf bytes.Buffer
-	if _, err := l.Client.DownloadFile(ctx, item.ID, &buf); err != nil {
+	if _, err := l.fileStore().Download(ctx, item.ID, &buf); err != nil {
 		return "", err
 	}
 	return preview.FileBytesToMarkdown(name, buf.Bytes())