chore: stop lint rules that do not model runes from reporting

no-unused-expressions is off for Svelte files: a bare `book;` in an $effect
declares a dependency. The URL and Set instances it flagged are local
temporaries, marked individually. The one real finding was visibleApiKeys,
genuine reactive state cloned on every mutation to force an update; it is a
SvelteSet now.

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

@@ -116,6 +116,7 @@ export class BookshelfState {
 		if (!bookshelves) return;
 
 		// Create a Set of shelf IDs that need updating for efficient lookup
+		// eslint-disable-next-line svelte/prefer-svelte-reactivity -- local temporary, not reactive state
 		const shelfIdsToUpdate = new Set<number>();
 		books.forEach((book) => {
 			book.lists.forEach((shelf) => {