refactor(document): extract shared DocumentOption type and createDocumentTypeahead factory

DocumentPickerDropdown and DocumentMultiSelect had identical createTypeahead
configs, fetch logic, and formatDocLabel helpers. Extracted to
documentTypeahead.ts; all four consumers import from the shared module.

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

frontend/src/lib/document/DocumentPickerDropdown.svelte

@@ -1,16 +1,11 @@
 <script lang="ts">
-import type { components } from '$lib/generated/api';
 import { m } from '$lib/paraglide/messages.js';
 import { clickOutside } from '$lib/shared/actions/clickOutside';
-import { createTypeahead } from '$lib/shared/hooks/useTypeahead.svelte';
-import { formatDocumentDate, type DatePrecision } from '$lib/shared/utils/documentDate';
-import { getLocale } from '$lib/paraglide/runtime.js';
-
-type DocumentListItem = components['schemas']['DocumentListItem'];
-type DocumentOption = Pick<
-	DocumentListItem,
-	'id' | 'title' | 'documentDate' | 'metaDatePrecision' | 'metaDateEnd'
->;
+import {
+	createDocumentTypeahead,
+	formatDocumentOption,
+	type DocumentOption
+} from './documentTypeahead';
 
 interface Props {
 	alreadyAddedIds?: Set<string>;
@@ -26,20 +21,7 @@ let {
 
 const listboxId = 'doc-picker-listbox';
 
-const picker = createTypeahead<DocumentOption>({
-	fetchUrl: (q) =>
-		fetch(`/api/documents/search?q=${encodeURIComponent(q)}&size=10`)
-			.then((r) => r.json())
-			.then((b: { items: DocumentListItem[] }) =>
-				b.items.map((it) => ({
-					id: it.id,
-					title: it.title,
-					documentDate: it.documentDate,
-					metaDatePrecision: it.metaDatePrecision,
-					metaDateEnd: it.metaDateEnd
-				}))
-			)
-});
+const picker = createDocumentTypeahead();
 
 let inputValue = $state('');
 
@@ -59,18 +41,6 @@ function handleSelect(doc: DocumentOption) {
 	picker.close();
 	onSelect(doc);
 }
-
-function formatDocLabel(doc: DocumentOption): string {
-	if (!doc.documentDate) return doc.title;
-	const label = formatDocumentDate(
-		doc.documentDate,
-		doc.metaDatePrecision as DatePrecision,
-		doc.metaDateEnd,
-		null,
-		getLocale()
-	);
-	return `${doc.title} · ${label}`;
-}
 </script>
 
 <div use:clickOutside onclickoutside={() => picker.close()} class="relative">
@@ -113,7 +83,7 @@ function formatDocLabel(doc: DocumentOption): string {
 								: 'cursor-pointer hover:bg-muted focus:bg-muted focus:outline-none'
 						].join(' ')}
 					>
-						{formatDocLabel(doc)}
+						{formatDocumentOption(doc)}
 						{#if disabled}
 							<span class="sr-only">{m.journey_already_added()}</span>
 						{/if}