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/lib/nav/AppShell.svelte

@@ -4,7 +4,7 @@
 	import TabletNavBar from './TabletNavBar.svelte';
 	import DesktopSidebar from './DesktopSidebar.svelte';
 
-	let { appName, householdName, children }: { appName: string; householdName: string; children: Snippet } = $props();
+	let { appName, householdName, children }: { appName: string; householdName: string; children?: Snippet } = $props();
 </script>
 
 <div class="flex min-h-screen bg-[var(--color-page)]">