feat(documents): add fetchDensity helper and /documents/+page.ts (#385)

The density data is fetched only on tablet/desktop (sm:+ breakpoint) and
when ?view=calendar is not set — mobile users and the future calendar view
(#386) skip the request entirely. Lives in +page.ts (client-side) so the
matchMedia gate can run in the browser; +page.server.ts continues to handle
the document search.

Non-ok responses and network failures degrade to an empty bucket list
rather than throwing, so the document list keeps rendering.

5 unit tests cover the gating + graceful degradation paths.

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

frontend/src/lib/document/timeline.ts

@@ -1,6 +1,16 @@
 import type { components } from '$lib/generated/api';
 
 type MonthBucket = components['schemas']['MonthBucket'];
+type DocumentDensityResult = components['schemas']['DocumentDensityResult'];
+
+export type DensityState = {
+	density: MonthBucket[] | null;
+	minDate: string | null;
+	maxDate: string | null;
+};
+
+const SKIP: DensityState = { density: null, minDate: null, maxDate: null };
+const EMPTY: DensityState = { density: [], minDate: null, maxDate: null };
 
 export function monthBoundaryFrom(yearMonth: string): string {
 	return `${yearMonth}-01`;
@@ -45,3 +55,30 @@ export function fillDensityGaps(
 	const counts = new Map(buckets.map((b) => [b.month, b.count]));
 	return sequence.map((month) => ({ month, count: counts.get(month) ?? 0 }));
 }
+
+/**
+ * Loads the density data for the timeline widget. Mobile (sm: breakpoint and below)
+ * and calendar view both skip the request entirely — the widget isn't rendered
+ * there. A non-ok response or network failure degrades to an empty bucket list
+ * instead of throwing, so the document list page keeps rendering.
+ */
+export async function fetchDensity(
+	fetch: typeof globalThis.fetch,
+	view: string | null,
+	isDesktop: boolean
+): Promise<DensityState> {
+	if (!isDesktop || view === 'calendar') return SKIP;
+
+	try {
+		const response = await fetch('/api/documents/density');
+		if (!response.ok) return EMPTY;
+		const body = (await response.json()) as DocumentDensityResult;
+		return {
+			density: body.buckets,
+			minDate: body.minDate ?? null,
+			maxDate: body.maxDate ?? null
+		};
+	} catch {
+		return EMPTY;
+	}
+}