feat(storybook): create ThemeDecorator to support themeManager logic in storybook

.storybook/ThemeDecorator.svelte

@@ -0,0 +1,30 @@
+<!--
+    Component: ThemeDecorator
+    Storybook decorator that initializes ThemeManager for theme-related stories.
+    Ensures theme management works correctly in Storybook's iframe environment.
+-->
+<script lang="ts">
+import { themeManager } from '$features/ChangeAppTheme';
+import {
+    onDestroy,
+    onMount,
+} from 'svelte';
+
+interface Props {
+    children: import('svelte').Snippet;
+}
+
+let { children }: Props = $props();
+
+// Initialize themeManager on mount
+onMount(() => {
+    themeManager.init();
+});
+
+// Clean up themeManager when story unmounts
+onDestroy(() => {
+    themeManager.destroy();
+});
+</script>
+
+{@render children()}