feat: draw generated covers where a book has none

frontend/src/lib/components/forms/merge-books/merge-books.svelte

@@ -10,6 +10,7 @@
 	import { Textarea } from '$lib/components/ui/textarea/index.js';
 	import { Badge } from '$lib/components/ui/badge/index.js';
 	import BookImage from '$lib/components/view/book-image.svelte';
+	import GeneratedCover from '$lib/components/view/generated-cover.svelte';
 	import { mergeBooks } from '$lib/api/book.remote';
 	import type { Book } from '$lib/schema';
 
@@ -132,6 +133,10 @@
 	<span class="{size} shrink-0 overflow-hidden rounded-sm border bg-muted">
 		{#if book.cover_image}
 			<BookImage src="/api/{book.cover_image}" class="h-full w-full object-cover" />
+		{:else}
+			<!-- Drawn rather than left blank, so the rail tells two coverless books
+			     apart the same way the shelves do. -->
+			<GeneratedCover {book} />
 		{/if}
 	</span>
 {/snippet}

frontend/src/lib/components/view/generated-cover.svelte

@@ -1,7 +1,14 @@
 <script lang="ts">
-	import type { Book } from '$lib/schema';
-
-	let { book, class: className = '' }: { book: Book; class?: string } = $props();
+	// Typed by what is drawn rather than by `Book`: the duplicates review holds
+	// candidates that carry a title and author names without a whole record, and a
+	// `Book` satisfies this shape anyway.
+	let {
+		book,
+		class: className = ''
+	}: {
+		book: { title: string; authors?: { name: string }[] | null };
+		class?: string;
+	} = $props();
 
 	/**
 	 * Bookcloth tones rather than a point on the hue wheel.