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/routes/(root)/(library)/+layout.svelte

@@ -1,7 +1,7 @@
 <script lang="ts">
 	import BookDelete from '$lib/components/forms/book-delete.svelte';
 	import { BookEdit } from '$lib/components/forms/edit-book';
-	import { getBookOperationsState, setBookOperationsState } from '$lib/state/bookOperations.svelte';
+	import { getBookOperationsState } from '$lib/state/bookOperations.svelte';
 	import { getLibraryState } from '$lib/state/library.svelte';
 	import { setBookSelectionState } from '$lib/state/bookSelection.svelte';
 	import type { Snippet } from 'svelte';