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/(public)/+layout.svelte

@@ -0,0 +1,12 @@
+<script>
+	let { children } = $props();
+</script>
+
+<div class="flex min-h-screen">
+	<div class="hidden md:flex md:w-1/2 bg-[var(--green)] items-center justify-center">
+		<span class="font-[var(--font-display)] text-4xl text-white font-medium">Mealprep</span>
+	</div>
+	<div class="flex-1 flex items-center justify-center p-6">
+		{@render children()}
+	</div>
+</div>