refactor(filters): replace filter/sort store singletons with lazy accessors

Convert appliedFilterStore, availableFilterStore and sortStore from eager
module-level singletons to getAppliedFilterStore/getAvailableFilterStore/
getSortStore lazy accessors (+ __reset* helpers for tests), so the
availableFilterStore QueryObserver is built on first use rather than at import.

Update barrels, the startFilterBindings bridge, and all consumers. Reactive
reads in components are wrapped in $derived; two-way bind:value targets resolve
the accessor once and bind directly (a $derived is read-only).

src/widgets/ComparisonView/ui/Search/Search.svelte

@@ -5,8 +5,10 @@
     propagates the value into fontCatalogStore.
 -->
 <script lang="ts">
-import { appliedFilterStore } from '$features/FilterAndSortFonts';
+import { getAppliedFilterStore } from '$features/FilterAndSortFonts';
 import { SearchBar } from '$shared/ui';
+
+const appliedFilterStore = getAppliedFilterStore();
 </script>
 
 <div class="p-6 border-b border-subtle">

src/widgets/ComparisonView/ui/Search/Search.svelte.test.ts

@@ -1,4 +1,4 @@
-import { appliedFilterStore } from '$features/FilterAndSortFonts';
+import { getAppliedFilterStore } from '$features/FilterAndSortFonts';
 import {
     render,
     screen,
@@ -7,7 +7,7 @@ import Search from './Search.svelte';
 
 describe('Search', () => {
     beforeEach(() => {
-        appliedFilterStore.queryValue = '';
+        getAppliedFilterStore().queryValue = '';
     });
 
     describe('Rendering', () => {
@@ -24,7 +24,7 @@ describe('Search', () => {
 
     describe('Value binding', () => {
         it('reflects appliedFilterStore.queryValue as initial value', () => {
-            appliedFilterStore.queryValue = 'Inter';
+            getAppliedFilterStore().queryValue = 'Inter';
             render(Search);
             expect(screen.getByRole('textbox')).toHaveValue('Inter');
         });