refactor(GetFonts): rename filters/filterManager to available/appliedFilterStore

The 'filters' + 'filterManager' pair didn't reveal the schema-vs-selection
split. Rename to reflect the actual roles:

- FiltersStore / filtersStore       → AvailableFilterStore / availableFilterStore
- createFilterManager / FilterManager → createAppliedFilterStore / AppliedFilterStore
- filterManager singleton            → appliedFilterStore
- mapManagerToParams                 → mapAppliedFiltersToParams

Directories and file basenames follow the new singleton names. Public
barrel signature updated; all consumers (Search, FontSearch, Filters,
FilterControls) point at the new identifiers.

src/widgets/FontSearch/ui/FontSearch/FontSearch.svelte

@@ -6,7 +6,7 @@
 import {
     FilterControls,
     Filters,
-    filterManager,
+    appliedFilterStore,
 } from '$features/GetFonts';
 import { springySlideFade } from '$shared/lib';
 import {
@@ -58,7 +58,7 @@ function toggleFilters() {
             class="w-full"
             placeholder="Typeface Search"
             aria-label="Search typefaces"
-            bind:value={filterManager.queryValue}
+            bind:value={appliedFilterStore.queryValue}
             fullWidth
         />