feat(frontend): add DocumentThumbnail shared 60x84 tile component

Renders the document thumbnail with object-cover + object-top so
letter salutations stay visible, empty alt (title nearby is the
accessible name), loading=lazy, decoding=async, and dark:mix-blend-multiply
for dark mode. Falls back to a PDF icon when thumbnailKey is null —
legacy documents, unsupported content types, or transient failures
all land here.

Refs #307

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/components/DocumentThumbnail.svelte

@@ -0,0 +1,51 @@
+<script lang="ts">
+import type { components } from '$lib/generated/api';
+import { thumbnailUrl } from '$lib/thumbnails';
+
+type Doc = Pick<
+	components['schemas']['Document'],
+	'id' | 'thumbnailKey' | 'thumbnailGeneratedAt' | 'contentType'
+>;
+
+let { doc }: { doc: Doc } = $props();
+const url = $derived(thumbnailUrl(doc));
+</script>
+
+<!--
+	Fixed 60x84 tile used wherever a document row appears. When the backend has
+	generated a thumbnail we render it with `object-cover` + `object-top` so
+	letter salutations stay visible; otherwise we fall back to the file-type
+	icon so the row never shows an empty rectangle. Dark mode uses
+	`mix-blend-multiply` to keep bright paper scans from glaring against the
+	dark page background.
+-->
+<div
+	class="relative h-[84px] w-[60px] flex-shrink-0 overflow-hidden rounded-sm border border-line bg-white"
+>
+	{#if url}
+		<img
+			src={url}
+			alt=""
+			class="h-full w-full object-cover object-top dark:mix-blend-multiply"
+			loading="lazy"
+			decoding="async"
+		/>
+	{:else}
+		<div class="flex h-full w-full items-center justify-center text-ink-3" aria-hidden="true">
+			<svg
+				xmlns="http://www.w3.org/2000/svg"
+				fill="none"
+				viewBox="0 0 24 24"
+				stroke-width="1.5"
+				stroke="currentColor"
+				class="h-6 w-6"
+			>
+				<path
+					stroke-linecap="round"
+					stroke-linejoin="round"
+					d="M19.5 14.25v-2.625a3.375 3.375 0 0 0-3.375-3.375h-1.5A1.125 1.125 0 0 1 13.5 7.125v-1.5a3.375 3.375 0 0 0-3.375-3.375H8.25m0 12.75h7.5m-7.5 3H12M10.5 2.25H5.625c-.621 0-1.125.504-1.125 1.125v17.25c0 .621.504 1.125 1.125 1.125h12.75c.621 0 1.125-.504 1.125-1.125V11.25a9 9 0 0 0-9-9Z"
+				/>
+			</svg>
+		</div>
+	{/if}
+</div>