chore: clear the mechanical lint and type findings

Dead imports and locals removed, each blocks keyed, `any` narrowed to unknown.
Two context setters kept their calls and lost only the unused binding; the
settings redirect no longer awaits a parent whose data it discards. A leading
underscore now marks a binding that only holds a position.

frontend/src/lib/state/bookCollection.svelte.ts

@@ -1,5 +1,5 @@
 import { getContext, setContext } from 'svelte';
-import { goto, pushState, replaceState } from '$app/navigation';
+import { goto, replaceState } from '$app/navigation';
 import type { Book, DynamicFilterData, FilterOption, PaginatedResponse } from '$lib/schema';
 import { page } from '$app/state';
 import { BookOperationsState } from './bookOperations.svelte';