feat(shell): add route groups, layout server load, redirect, and placeholder pages

- (app) group with AppShell layout, loads user/household from locals
- (public) group with full-viewport split layout, /login placeholder
- Root / redirects to /planner for authenticated users
- Placeholder stubs for planner, recipes, shopping, settings, members

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

frontend/src/routes/(app)/+layout.server.ts

@@ -0,0 +1,8 @@
+import type { LayoutServerLoad } from './$types';
+
+export const load: LayoutServerLoad = async ({ locals }) => {
+	return {
+		benutzer: locals.benutzer!,
+		haushalt: locals.haushalt!
+	};
+};