fix: replace deprecated EditorSnackbars with SnackbarNotices

`wp.editor.EditorSnackbars` is deprecated since @wordpress/editor v7.0
and will be removed in v7.2. Replace with `wp.notices.SnackbarNotices`,
using a project-owned `gutenberg-kit-layout__snackbar` class and moving
the positioning styles to layout/style.scss.

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

Author: dcalhoun

src/components/editor/style.scss

@@ -37,12 +37,3 @@ button {
 	font-size: inherit;
 	font-weight: inherit;
 }
-
-.gutenberg-kit-editor .components-editor-notices__snackbar {
-	bottom: 58px;
-	left: 0;
-	padding-right: 8px;
-	padding-left: 8px;
-	position: fixed;
-	right: 0;
-}

src/components/layout/index.jsx

@@ -1,11 +1,8 @@
 /**
  * WordPress dependencies
  */
-import {
-	EditorSnackbars,
-	ErrorBoundary,
-	AutosaveMonitor,
-} from '@wordpress/editor';
+import { ErrorBoundary, AutosaveMonitor } from '@wordpress/editor';
+import { SnackbarNotices } from '@wordpress/notices';
 
 /**
  * Internal dependencies
@@ -32,7 +29,7 @@ export default function Layout( props ) {
 			<OfflineIndicator />
 			<AutosaveMonitor autosave={ onEditorContentChanged } />
 			<Editor { ...editorProps }>
-				<EditorSnackbars />
+				<SnackbarNotices className="gutenberg-kit-layout__snackbar" />
 			</Editor>
 			<EditorLoadNotice
 				className="gutenberg-kit-layout__load-notice"

src/components/layout/style.scss

@@ -1,3 +1,12 @@
+.gutenberg-kit-layout__snackbar {
+	bottom: 58px;
+	left: 0;
+	padding-right: 8px;
+	padding-left: 8px;
+	position: fixed;
+	right: 0;
+}
+
 .gutenberg-kit-layout__load-notice {
 	bottom: 62px;
 	left: 16px;