feat(timeline): add /zeitstrahl route, SSR load, and nav link

SSR-first load fetches GET /api/timeline via createApiClient (auth cookie
forwarded), no query params for the global view (REQ-001), returns { timeline }
with no client-side fetch (REQ-002); 401 -> /login, any other non-ok ->
error(status, getErrorMessage(...)), never raw JSON, no PII logged (REQ-022).
The page renders <TimelineView> under the layout's <main>. Adds the Zeitstrahl
nav link (desktop + mobile) and 'timeline' to the eslint routes boundary
allow-list so the route may import the domain.

Refs #779
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -0,0 +1,16 @@
+<script lang="ts">
+import * as m from '$lib/paraglide/messages.js';
+import TimelineView from '$lib/timeline/TimelineView.svelte';
+import type { PageData } from './$types';
+
+let { data }: { data: PageData } = $props();
+</script>
+
+<svelte:head>
+	<title>{m.timeline_heading()}</title>
+</svelte:head>
+
+<div class="mx-auto max-w-5xl px-4 py-8">
+	<h1 class="mb-8 font-serif text-2xl font-bold text-brand-navy">{m.timeline_heading()}</h1>
+	<TimelineView timeline={data.timeline} />
+</div>