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/components/view/book-image.svelte

@@ -1,8 +1,10 @@
 <script>
 	let { src, fallback = '/images/default_cover.jpg', class: className = '' } = $props();
 
-	async function handleError(e) {
-		e.target.src = fallback;
+	/** @param {Event} e */
+	function handleError(e) {
+		const img = /** @type {HTMLImageElement} */ (e.currentTarget);
+		img.src = fallback;
 	}
 </script>