feat(documents): wire TimelineDensityFilter into /documents/+page (#385)

Mounts the timeline above the result count, hidden on mobile via
\`hidden sm:block\` (defense-in-depth — +page.ts already gates the fetch).
The component's onchange callback updates local from/to and triggers
the existing search reload, so timeline selection composes with the
SearchFilterBar's other filters via AND semantics for free.

3 new page-level integration tests cover: widget renders when density
present, hides when null, and bar click navigates with correct
from/to URL params.

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

frontend/src/routes/documents/+page.svelte

@@ -7,6 +7,7 @@ import SearchFilterBar from '../SearchFilterBar.svelte';
 import DocumentList from '../DocumentList.svelte';
 import Pagination from '$lib/shared/primitives/Pagination.svelte';
 import BulkSelectionBar from '$lib/document/BulkSelectionBar.svelte';
+import TimelineDensityFilter from '$lib/document/TimelineDensityFilter.svelte';
 import { bulkSelectionStore } from '$lib/document/bulkSelection.svelte';
 import { getErrorMessage, parseBackendError } from '$lib/shared/errors';
 import * as m from '$lib/paraglide/messages.js';
@@ -234,6 +235,21 @@ $effect(() => {
 		onblur={() => (qFocused = false)}
 	/>
 
+	<div class="mt-3 mb-4 hidden sm:block">
+		<TimelineDensityFilter
+			density={data.density}
+			minDate={data.minDate}
+			maxDate={data.maxDate}
+			from={from}
+			to={to}
+			onchange={(event) => {
+				from = event.from;
+				to = event.to;
+				triggerSearch();
+			}}
+		/>
+	</div>
+
 	<div class="mb-3 flex items-center justify-between gap-4">
 		<p class="font-sans text-base text-ink-2">
 			{#if data.totalElements > 0}{m.docs_result_count({ count: data.totalElements })}{/if}