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/AppNav.svelte

@@ -78,6 +78,16 @@ function handleOverlayKeydown(event: KeyboardEvent) {
 		>
 			{m.nav_geschichten()}
 		</a>
+
+		<a
+			href="/zeitstrahl"
+			class="my-2 inline-flex items-center px-3 font-sans text-xs font-bold tracking-widest uppercase transition-colors focus:outline-none focus-visible:rounded focus-visible:ring-2 focus-visible:ring-focus-ring
+                   {page.url.pathname.startsWith('/zeitstrahl')
+				? 'border-b-2 border-accent text-white'
+				: 'text-white/70 hover:text-white'}"
+		>
+			{m.nav_zeitstrahl()}
+		</a>
 		{#if isAdmin}
 			<a
 				href="/admin"
@@ -190,6 +200,16 @@ function handleOverlayKeydown(event: KeyboardEvent) {
 					{m.nav_geschichten()}
 				</a>
 
+				<a
+					href="/zeitstrahl"
+					class="block flex min-h-[44px] w-full items-center px-4 py-3 font-sans text-sm font-bold tracking-widest uppercase transition-colors focus:outline-none focus-visible:ring-2 focus-visible:ring-focus-ring focus-visible:ring-inset
+                           {page.url.pathname.startsWith('/zeitstrahl')
+						? 'bg-accent-bg text-ink'
+						: 'text-ink-2 hover:bg-muted hover:text-ink'}"
+				>
+					{m.nav_zeitstrahl()}
+				</a>
+
 				{#if isAdmin}
 					<a
 						href="/admin"