refactor: adopt createSingleton across the remaining stores

Replace the hand-rolled let _x / getX / __resetX boilerplate with the
createSingleton helper in all nine remaining singleton stores. Exposed
accessor names (getX, __resetX) are unchanged, so consumers and specs are
unaffected. Teardown wired to each stores destroy() where it has one
(fontCatalog, fontLifecycle, typography, availableFilter, theme, layout,
scrollBreadcrumbs); sort and appliedFilter have no teardown. Also merges
layoutStores duplicate $shared/lib imports.

src/entities/Font/model/store/fontLifecycleManager/fontLifecycleManager.svelte.ts

@@ -1,3 +1,4 @@
+import { createSingleton } from '$shared/lib/helpers/createSingleton/createSingleton';
 import { SvelteMap } from 'svelte/reactivity';
 import {
     type FontLoadRequestConfig,
@@ -419,18 +420,16 @@ export class FontLifecycleManager {
     }
 }
 
-let _fontLifecycleManager: FontLifecycleManager | undefined;
-
 /**
  * App-wide font lifecycle manager, created on first access. Lazy so its
  * AbortController / FontFace bookkeeping isn't set up at module load.
  */
-export function getFontLifecycleManager(): FontLifecycleManager {
-    return (_fontLifecycleManager ??= new FontLifecycleManager());
-}
+const fontLifecycleManager = createSingleton(
+    () => new FontLifecycleManager(),
+    instance => instance.destroy(),
+);
+
+export const getFontLifecycleManager = fontLifecycleManager.get;
 
 // test-only reset, so specs don't share loaded-font/eviction state
-export function __resetFontLifecycleManager() {
-    _fontLifecycleManager?.destroy();
-    _fontLifecycleManager = undefined;
-}
+export const __resetFontLifecycleManager = fontLifecycleManager.reset;