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/features/Breadcrumb/model/store/scrollBreadcrumbsStore.svelte.ts

@@ -1,3 +1,5 @@
+import { createSingleton } from '$shared/lib/helpers/createSingleton/createSingleton';
+
 /**
  * Scroll-based breadcrumb tracking store
  *
@@ -279,17 +281,15 @@ export function createScrollBreadcrumbsStore(): ScrollBreadcrumbsStore {
     return new ScrollBreadcrumbsStore();
 }
 
-let _scrollBreadcrumbsStore: ScrollBreadcrumbsStore | undefined;
-
 /**
  * App-wide scroll breadcrumbs store, created on first access.
  */
-export function getScrollBreadcrumbsStore(): ScrollBreadcrumbsStore {
-    return (_scrollBreadcrumbsStore ??= createScrollBreadcrumbsStore());
-}
+const scrollBreadcrumbsStore = createSingleton(
+    () => createScrollBreadcrumbsStore(),
+    instance => instance.destroy(),
+);
+
+export const getScrollBreadcrumbsStore = scrollBreadcrumbsStore.get;
 
 // test-only reset, so specs don't share observer/scroll state
-export function __resetScrollBreadcrumbsStore() {
-    _scrollBreadcrumbsStore?.destroy();
-    _scrollBreadcrumbsStore = undefined;
-}
+export const __resetScrollBreadcrumbsStore = scrollBreadcrumbsStore.reset;