fix: show error for duplicate folder name + add client-side validation

CreateDirectory now returns an error when the folder already exists
instead of silently succeeding. JS validates folder name format
(alphanumeric + underscore, max 32 chars) before sending the request.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

controller/internal/storage/attach_linux.go

@@ -148,7 +148,7 @@ func CreateDirectory(basePath, name string) (string, error) {
 	// Check if already exists
 	if fi, err := os.Stat(targetPath); err == nil {
 		if fi.IsDir() {
-			return targetPath, nil // already exists, idempotent
+			return "", fmt.Errorf("a mappa már létezik: %s", name)
 		}
 		return "", fmt.Errorf("a cél már létezik és nem mappa")
 	}