fix: preserve WebDAV MOVE destination precedence

chfs-ng/internal/api/server.go

@@ -1437,10 +1437,19 @@ func (f *webdavAuthFS) Move(ctx context.Context, name, dest string, opts *webdav
 	// This matters because MOVE to an occupied destination returns 412 even
 	// when the source has disappeared, while a missing source with a free
 	// destination is reported as 403.
-	if opts == nil || !opts.NoOverwrite {
-		if err := f.authorizeOverwrite(ctx, dest); err != nil {
-			return false, err
+	destExists := false
+	if _, err := f.inner.Stat(ctx, dest); err == nil {
+		destExists = true
+	} else if !isWebDAVNotFound(err) {
+		return false, err
+	}
+	if destExists {
+		if opts == nil || !opts.NoOverwrite {
+			if err := f.authorizeOverwrite(ctx, dest); err != nil {
+				return false, err
+			}
 		}
+		return false, webdav.NewHTTPError(http.StatusPreconditionFailed, fmt.Errorf("destination exists"))
 	}
 	if _, err := f.inner.Stat(ctx, name); err != nil {
 		if os.IsNotExist(err) {
@@ -1454,6 +1463,13 @@ func (f *webdavAuthFS) Move(ctx context.Context, name, dest string, opts *webdav
 	return f.inner.Move(ctx, name, dest, opts)
 }
 
+func isWebDAVNotFound(err error) bool {
+	if err == nil {
+		return false
+	}
+	return os.IsNotExist(err) || strings.HasPrefix(err.Error(), "404 Not Found:")
+}
+
 func (f *webdavAuthFS) authorizeOverwrite(ctx context.Context, dest string) error {
 	info, err := f.inner.Stat(ctx, dest)
 	if err != nil {