feat(nav): add AppShell layout with breakpoint-switched navigation

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

frontend/src/lib/nav/AppShell.svelte

@@ -0,0 +1,19 @@
+<script lang="ts">
+	import type { Snippet } from 'svelte';
+	import MobileTabBar from './MobileTabBar.svelte';
+	import TabletNavBar from './TabletNavBar.svelte';
+	import DesktopSidebar from './DesktopSidebar.svelte';
+
+	let { appName, householdName, children }: { appName: string; householdName: string; children: Snippet } = $props();
+</script>
+
+<div class="flex min-h-screen bg-[var(--color-page)]">
+	<DesktopSidebar {appName} {householdName} />
+	<div class="flex flex-1 flex-col">
+		<TabletNavBar />
+		<main class="flex-1">
+			{@render children?.()}
+		</main>
+		<MobileTabBar />
+	</div>
+</div>