fix: type the proxy handlers and the delete callback

The RequestHandler import sat commented out while all four handlers were
annotated with it, so every destructured parameter was an implicit any --
24 of the 30 svelte-check errors. deleteFn returned `{}`, which void is not
assignable to.

frontend/src/lib/components/forms/book-delete.svelte

@@ -13,7 +13,7 @@
 	}: {
 		open: boolean;
 		title?: string;
-		deleteFn: (deleteFiles: boolean) => {};
+		deleteFn: (deleteFiles: boolean) => void | Promise<void>;
 	} = $props();
 
 	const selectedState = getBookSelectionState();