fix: use nullish coalescing assignment (??=) for feature flag setup

Use ??= instead of ||= for initializing Vaadin.featureFlags objects.
This is more semantically correct as it only assigns when the value
is nullish (null/undefined) rather than any falsy value, preventing
accidental overwrites of existing objects.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: Artur-

dev/breadcrumb.html

@@ -7,8 +7,8 @@
   <script type="module" src="./common.js"></script>
   <script type="module">
     // Enable experimental breadcrumb component
-    window.Vaadin ||= {};
-    window.Vaadin.featureFlags ||= {};
+    window.Vaadin ??= {};
+    window.Vaadin.featureFlags ??= {};
     window.Vaadin.featureFlags.breadcrumbComponent = true;
 
     import '@vaadin/breadcrumb/vaadin-breadcrumb.js';

packages/breadcrumb/test/breadcrumb.test.js

@@ -2,8 +2,8 @@ import { expect } from '@vaadin/chai-plugins';
 import { fixtureSync, nextFrame } from '@vaadin/testing-helpers';
 
 // Enable experimental breadcrumb component
-window.Vaadin ||= {};
-window.Vaadin.featureFlags ||= {};
+window.Vaadin ??= {};
+window.Vaadin.featureFlags ??= {};
 window.Vaadin.featureFlags.breadcrumbComponent = true;
 
 import '../vaadin-breadcrumb.js';