fix(documents): merge +page.server data into +page.ts return (#385)

SvelteKit's PageData type generation only picks up +page.ts return values
when both files exist, so the runtime-merged server data was invisible to
TypeScript and svelte-check flagged every q/from/to/etc access in
+page.svelte. Spreading data into the +page.ts return restores the merge
at the type level. No runtime behaviour change.

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

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

@@ -2,8 +2,9 @@ import { browser } from '$app/environment';
 import { fetchDensity } from '$lib/document/timeline';
 import type { PageLoad } from './$types';
 
-export const load: PageLoad = async ({ url, fetch }) => {
+export const load: PageLoad = async ({ url, fetch, data }) => {
 	const view = url.searchParams.get('view');
 	const isDesktop = browser && window.matchMedia('(min-width: 640px)').matches;
-	return await fetchDensity(fetch, view, isDesktop);
+	const density = await fetchDensity(fetch, view, isDesktop);
+	return { ...data, ...density };
 };