feat(briefwechsel): add ConversationThumbnail with aspect + page badge

Reads thumbnailAspect from the backend and swaps between a 120×168
portrait tile and a 168×120 landscape tile so postcards and photos
don't get cropped into a portrait frame. Shows a page-count badge
top-right for multi-page PDFs, and a pulsing skeleton while the
async thumbnail job hasn't run yet. URL assembly goes through the
existing thumbnailUrl helper so cache-busting stays consistent
with DocumentThumbnail.

Refs #305

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

frontend/src/lib/components/ConversationThumbnail.svelte

@@ -0,0 +1,48 @@
+<script lang="ts">
+import { thumbnailUrl } from '$lib/thumbnails';
+
+type Doc = {
+	id: string;
+	thumbnailKey?: string;
+	thumbnailGeneratedAt?: string;
+	thumbnailAspect?: 'PORTRAIT' | 'LANDSCAPE';
+	pageCount?: number;
+};
+
+let { doc }: { doc: Doc } = $props();
+
+const url = $derived(thumbnailUrl(doc));
+const aspect = $derived(doc.thumbnailAspect ?? 'PORTRAIT');
+const pageCount = $derived(doc.pageCount ?? 1);
+const tileClass = $derived(aspect === 'LANDSCAPE' ? 'h-[120px] w-[168px]' : 'h-[168px] w-[120px]');
+</script>
+
+<div
+	data-testid="conv-thumb-tile"
+	data-aspect={aspect}
+	class="relative {tileClass} 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
+			data-testid="conv-thumb-skeleton"
+			class="h-full w-full bg-line/60 motion-safe:animate-pulse"
+			aria-hidden="true"
+		></div>
+	{/if}
+
+	{#if pageCount > 1}
+		<span
+			data-testid="conv-thumb-page-badge"
+			class="absolute top-1 right-1 rounded-full bg-primary/90 px-1.5 py-0.5 text-xs leading-none font-bold text-surface"
+			>{pageCount}</span
+		>
+	{/if}
+</div>