Add a redirect for bare /admin to /admin/settings

Only /admin/login and /admin/settings existed as actual pages, so navigating
to /admin itself 404'd regardless of ADMIN_PANEL_ENABLED. Settings already
redirects to login on a 401, so that's the sensible default landing spot.

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

@@ -0,0 +1 @@
+<!-- Never actually renders — +page.ts's load always redirects to /admin/settings before this displays. -->

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

@@ -0,0 +1,9 @@
+import { redirect } from '@sveltejs/kit';
+import type { PageLoad } from './$types';
+
+// There's no page at exactly /admin — only /admin/login and /admin/settings — so a
+// bare visit here would otherwise 404. Settings itself already redirects to login on
+// a 401, so sending everyone through there is the one sensible default landing spot.
+export const load: PageLoad = async () => {
+	throw redirect(302, '/admin/settings');
+};