refactor(font): absorb FetchFontsByIds store into Font entity

FetchFontsByIds was data-access for the Font aggregate wearing a feature
costume: a single FontsByIdsStore (BaseQueryStore over Font's proxy API),
no UI, consumed only by comparisonStore. Move it beside fontCatalogStore
in the Font entity, collapse its deep $entities/Font imports to relative
paths, expose via the Font public API, and delete the feature slice.

src/entities/Font/model/store/index.ts

@@ -7,3 +7,6 @@ export {
     FontCatalogStore,
     fontCatalogStore,
 } from './fontCatalogStore/fontCatalogStore.svelte';
+
+// Batch fetch by IDs (detail-cache seeding)
+export { FontsByIdsStore } from './fontsByIdsStore/fontsByIdsStore.svelte';