refactor: move document domain core to lib/document/

Moves ~25 components, utils (search, filename, groupDocuments,
documentStatusLabel, validateFile), bulkSelection store, and
TranscriptionSection sub-component. Fixes broken relative imports.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/document/DocumentThumbnail.svelte

@@ -0,0 +1,56 @@
+<script lang="ts">
+import type { components } from '$lib/generated/api';
+
+type Doc = Pick<components['schemas']['Document'], 'id' | 'thumbnailUrl' | 'contentType'>;
+
+let { doc, size = 'sm' }: { doc: Doc; size?: 'sm' | 'lg' } = $props();
+const url = $derived(doc.thumbnailUrl ?? null);
+
+const containerClass = $derived(
+	size === 'lg'
+		? 'relative h-[168px] w-[120px] flex-shrink-0 overflow-hidden rounded-sm border border-line bg-white'
+		: 'relative h-[84px] w-[60px] flex-shrink-0 overflow-hidden rounded-sm border border-line bg-white'
+);
+const iconClass = $derived(size === 'lg' ? 'h-16 w-16' : 'h-8 w-8');
+</script>
+
+<!--
+	Fixed-aspect (5:7, ≈A4) tile used wherever a document row appears. `sm` is
+	60×84 (compact rows, person sublists); `lg` is 120×168 (main document list).
+	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={containerClass}>
+	{#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">
+			<!-- Generic document icon (heroicons document-text outline). Shown when the
+			     thumbnail hasn't been generated yet — applies equally to PDFs and to
+			     image scans, so we deliberately avoid a PDF-specific glyph here. -->
+			<svg
+				xmlns="http://www.w3.org/2000/svg"
+				fill="none"
+				viewBox="0 0 24 24"
+				stroke-width="1.5"
+				stroke="currentColor"
+				class={iconClass}
+			>
+				<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 0-3.375-3.375H8.25m2.25 0H5.625c-.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 M9 12.75h6M9 15.75h6M9 18.75h3"
+				/>
+			</svg>
+		</div>
+	{/if}
+</div>