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!
+	};
+};

frontend/src/routes/(app)/+layout.svelte

@@ -0,0 +1,9 @@
+<script lang="ts">
+	import AppShell from '$lib/nav/AppShell.svelte';
+
+	let { data, children } = $props();
+</script>
+
+<AppShell appName="Mealprep" householdName={data.haushalt.name}>
+	{@render children()}
+</AppShell>

frontend/src/routes/(app)/members/+page.svelte

@@ -0,0 +1 @@
+<h1 class="text-2xl font-medium p-6">Mitglieder</h1>

frontend/src/routes/(app)/planner/+page.svelte

@@ -0,0 +1 @@
+<h1 class="text-2xl font-medium p-6">Planer</h1>

frontend/src/routes/(app)/recipes/+page.svelte

@@ -0,0 +1 @@
+<h1 class="text-2xl font-medium p-6">Rezepte</h1>

frontend/src/routes/(app)/settings/+page.svelte

@@ -0,0 +1 @@
+<h1 class="text-2xl font-medium p-6">Einstellungen</h1>

frontend/src/routes/(app)/shopping/+page.svelte

@@ -0,0 +1 @@
+<h1 class="text-2xl font-medium p-6">Einkaufsliste</h1>