fix: align WebDAV missing-source MOVE semantics

chfs-ng/internal/api/server.go

@@ -1426,9 +1426,6 @@ func (f *webdavAuthFS) Move(ctx context.Context, name, dest string, opts *webdav
 	if err := f.allow(name, auth.Delete); err != nil {
 		return false, err
 	}
-	if err := f.checkTree(ctx, name, auth.Delete); err != nil {
-		return false, err
-	}
 	parent := path.Dir(webdavAuthPath(dest))
 	if parent == "." {
 		parent = ""
@@ -1436,11 +1433,24 @@ func (f *webdavAuthFS) Move(ctx context.Context, name, dest string, opts *webdav
 	if err := f.allow(parent, auth.Write); err != nil {
 		return false, err
 	}
+	// CHFS checks an existing destination before resolving a missing source.
+	// 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
 		}
 	}
+	if _, err := f.inner.Stat(ctx, name); err != nil {
+		if os.IsNotExist(err) {
+			return false, webdav.NewHTTPError(http.StatusForbidden, fmt.Errorf("source not found"))
+		}
+		return false, err
+	}
+	if err := f.checkTree(ctx, name, auth.Delete); err != nil {
+		return false, err
+	}
 	return f.inner.Move(ctx, name, dest, opts)
 }