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/library.svelte.ts

@@ -1,6 +1,6 @@
 import { resolve } from '$app/paths';
 import { browser } from '$app/environment';
-import { goto, invalidate } from '$app/navigation';
+import { goto } from '$app/navigation';
 import { page } from '$app/state';
 import { deleteLibrary } from '$lib/api';
 import type { Library } from '$lib/schema';