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());