feat: draw covers for books without one

Full-bleed serif title on a woven ground, drawn in the browser rather than
stored, sized with container queries so one component works from 36px to full
page. Colour is hashed into a list of bookcloth tones instead of the hue wheel.

The grid and search were falling back to default_cover.jpg because a missing
cover made the src "/api/undefined"; they now check for one first.

frontend/src/lib/components/layout/search-form.svelte

@@ -10,6 +10,7 @@
 	import { getLibraryState } from '$lib/state/library.svelte';
 	import type { PaginatedResponse, Book } from '$lib/schema';
 	import BookImage from '../view/book-image.svelte';
+	import GeneratedCover from '../view/generated-cover.svelte';
 	import { goto } from '$app/navigation';
 
 	const libraryState = getLibraryState();
@@ -104,10 +105,16 @@
 							class="cursor-pointer"
 						>
 							<div class="hover:bg-base-200 flex gap-4 p-3">
-								<BookImage
-									src="/api/{book.cover_image}"
-									class="w-24 rounded object-cover shadow-lg"
-								/>
+								{#if book.cover_image}
+									<BookImage
+										src="/api/{book.cover_image}"
+										class="w-24 rounded object-cover shadow-lg"
+									/>
+								{:else}
+									<div class="aspect-9/12 w-24 shrink-0 overflow-hidden rounded shadow-lg">
+										<GeneratedCover {book} />
+									</div>
+								{/if}
 								<div class="flex-top flex flex-col">
 									<span class="font-serif text-lg">{book.title}</span>
 									<span class=" text-md">{book.subtitle}</span>