feat: add ModalityMode enum (#22295)

Introduce ModalityMode with three modes: STRICT, VISUAL and MODELESS.

Part of: #22279

Author: tltv

flow-server/src/main/java/com/vaadin/flow/component/ModalityMode.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright 2000-2025 Vaadin Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+
+package com.vaadin.flow.component;
+
+import com.vaadin.flow.dom.DomListenerRegistration;
+
+/**
+ * Dialog modality mode.
+ */
+public enum ModalityMode {
+
+    /**
+     * Doesn’t use modality at all. No modality curtain, focus trap, pointer
+     * event blocking, and request blocking.
+     */
+    MODELESS,
+    /**
+     * A visual curtain dims the background behind the dialog, trapping focus
+     * and blocking pointer events. Background components can't receive requests
+     * unless explicitly enabled via low-level APIs like
+     * {@link DomListenerRegistration#allowInert()}.
+     */
+    STRICT,
+    /**
+     * Behaves like {@link #STRICT} except that request are not blocked.
+     */
+    VISUAL
+}