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();

frontend/src/routes/api/[...path]/+server.ts

@@ -1,7 +1,7 @@
 // src/routes/api/[...path]/+server.ts
 import { BACKEND_API_URL } from '$lib/server/config';
 import { json, error } from '@sveltejs/kit';
-// import type { RequestHandler } from './$types';
+import type { RequestHandler } from './$types';
 
 // Helper function to handle both JSON and file responses
 async function handleResponse(response: Response) {