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/view/book-thumbnail.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
 	import { resolve } from '$app/paths';
 	import BookImage from './book-image.svelte';
+	import GeneratedCover from './generated-cover.svelte';
 	import { Progress } from '$lib/components/ui/progress/index';
 	import { getLibraryState } from '$lib/state/library.svelte';
 	import { getBookSelectionState } from '$lib/state/bookSelection.svelte';
@@ -31,13 +32,30 @@
 			: ''}"
 		onclick={handleClick}
 	>
-		<BookImage
-			src="/api/{book.cover_image}"
-			class="h-full w-full rounded-sm object-cover transition-all duration-200 group-hover:brightness-50 {selected ||
-			darkened
-				? 'brightness-50'
-				: ''}"
-		/>
+		<!--
+			Checked rather than left to the image's onerror: with no cover_image the
+			src became "/api/undefined", 404'd, and fell back to the generic
+			default_cover.jpg — which is what made every coverless book look alike in
+			the grid.
+		-->
+		{#if book.cover_image}
+			<BookImage
+				src="/api/{book.cover_image}"
+				class="h-full w-full rounded-sm object-cover transition-all duration-200 group-hover:brightness-50 {selected ||
+				darkened
+					? 'brightness-50'
+					: ''}"
+			/>
+		{:else}
+			<div
+				class="h-full w-full transition-all duration-200 group-hover:brightness-50 {selected ||
+				darkened
+					? 'brightness-50'
+					: ''}"
+			>
+				<GeneratedCover {book} />
+			</div>
+		{/if}
 
 		<!-- Inside the anchor, which already clips to rounded-sm, so the bar
 		     follows the cover's corners instead of floating below it -->