refactor(theme): replace themeManager singleton with lazy getThemeManager

Convert the eager themeManager singleton to a getThemeManager() lazy accessor
(+ __resetThemeManager for tests), so the persistent-store subscription is set
up on first access rather than at module load. Update the barrel and consumers
(Layout init/destroy, ThemeSwitch, story, test).

src/app/ui/Layout.svelte

@@ -3,7 +3,7 @@
     Application shell with providers and page wrapper
 -->
 <script lang="ts">
-import { themeManager } from '$features/ChangeAppTheme';
+import { getThemeManager } from '$features/ChangeAppTheme';
 import G from '$shared/assets/G.svg';
 import { ResponsiveProvider } from '$shared/lib';
 import { cn } from '$shared/lib';
@@ -32,6 +32,8 @@ interface Props {
 
 let { children }: Props = $props();
 let fontsReady = $state(true);
+
+const themeManager = getThemeManager();
 const theme = $derived(themeManager.value);
 
 onMount(() => themeManager.init());

src/features/ChangeAppTheme/model/index.ts

@@ -1 +1 @@
-export { themeManager } from './store/ThemeManager/ThemeManager.svelte';
+export { getThemeManager } from './store/ThemeManager/ThemeManager.svelte';

src/features/ChangeAppTheme/model/store/ThemeManager/ThemeManager.svelte.ts

@@ -194,15 +194,26 @@ class ThemeManager {
     }
 }
 
+let _themeManager: ThemeManager | undefined;
+
 /**
- * Singleton theme manager instance
+ * App-wide theme manager, created on first access.
  *
- * Use throughout the app for consistent theme state.
+ * Lazy so its persistent-store subscription isn't set up at module load.
+ * Call init() on mount and destroy() on unmount (see Layout).
  */
-export const themeManager = new ThemeManager();
+export function getThemeManager(): ThemeManager {
+    return (_themeManager ??= new ThemeManager());
+}
+
+// test-only reset, so specs don't share persisted theme state
+export function __resetThemeManager() {
+    _themeManager?.destroy();
+    _themeManager = undefined;
+}
 
 /**
  * ThemeManager class exported for testing purposes
- * Use the singleton `themeManager` in application code.
+ * Use the `getThemeManager()` accessor in application code.
  */
 export { ThemeManager };

src/features/ChangeAppTheme/ui/ThemeSwitch/ThemeSwitch.stories.svelte

@@ -22,8 +22,9 @@ const { Story } = defineMeta({
 </script>
 
 <script lang="ts">
-import { themeManager } from '$features/ChangeAppTheme';
+import { getThemeManager } from '$features/ChangeAppTheme';
 
+const themeManager = getThemeManager();
 // Current theme state for display
 const currentTheme = $derived(themeManager.value);
 const themeSource = $derived(themeManager.source);

src/features/ChangeAppTheme/ui/ThemeSwitch/ThemeSwitch.svelte

@@ -8,10 +8,11 @@ import { IconButton } from '$shared/ui';
 import MoonIcon from '@lucide/svelte/icons/moon';
 import SunIcon from '@lucide/svelte/icons/sun';
 import { getContext } from 'svelte';
-import { themeManager } from '../../model';
+import { getThemeManager } from '../../model';
 
 const responsive = getContext<ResponsiveManager>('responsive');
 
+const themeManager = getThemeManager();
 const theme = $derived(themeManager.value);
 </script>
 

src/features/ChangeAppTheme/ui/ThemeSwitch/ThemeSwitch.svelte.test.ts

@@ -3,16 +3,25 @@ import {
     render,
     screen,
 } from '@testing-library/svelte';
-import { themeManager } from '../../model';
+import { afterEach } from 'vitest';
+import { getThemeManager } from '../../model';
+import { __resetThemeManager } from '../../model/store/ThemeManager/ThemeManager.svelte';
 import ThemeSwitch from './ThemeSwitch.svelte';
 
 const context = new Map([['responsive', { isMobile: false }]]);
 
 describe('ThemeSwitch', () => {
+    let themeManager: ReturnType<typeof getThemeManager>;
+
     beforeEach(() => {
+        themeManager = getThemeManager();
         themeManager.setTheme('light');
     });
 
+    afterEach(() => {
+        __resetThemeManager();
+    });
+
     describe('Rendering', () => {
         it('renders an icon button', () => {
             render(ThemeSwitch, { context });