feat(nav): add DesktopSidebar with logo, nav sections, and variety widget slot

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

frontend/src/lib/nav/DesktopSidebar.svelte

@@ -0,0 +1,42 @@
+<script lang="ts">
+	import { page } from '$app/stores';
+	import { desktopNavSections } from './nav';
+
+	let { appName, householdName }: { appName: string; householdName: string } = $props();
+</script>
+
+<aside
+	class="hidden lg:flex flex-col sticky top-0 h-screen w-[224px] min-w-[224px] border-r border-[var(--color-border)] bg-white"
+>
+	<div class="px-[18px] pt-[14px] pb-[14px] border-b border-[var(--color-border)]">
+		<div class="flex items-center gap-2">
+			<div class="w-[22px] h-[22px] bg-[var(--green)] rounded-[var(--radius-sm)]"></div>
+			<span class="font-[var(--font-display)] text-[15px] font-medium">{appName}</span>
+		</div>
+		<p class="text-[10px] text-[var(--color-text-muted)]">{householdName}</p>
+	</div>
+
+	<nav aria-label="Hauptnavigation" class="flex-1 overflow-y-auto px-2 py-1">
+		{#each desktopNavSections as section (section.title)}
+			<p
+				class="text-[8px] font-medium uppercase tracking-[0.1em] text-[var(--color-text-muted)] font-[var(--font-sans)] px-3 pt-4 pb-1"
+			>
+				{section.title}
+			</p>
+			{#each section.items as item (item.href)}
+				{@const active = $page.url.pathname.startsWith(item.href)}
+				<a
+					href={item.href}
+					aria-current={active ? 'page' : undefined}
+					class="px-3 py-[7px] text-[13px] font-[var(--font-sans)] rounded-[var(--radius-md)] flex items-center gap-2 {active
+						? 'bg-[var(--green-tint)] text-[var(--green-dark)] font-medium'
+						: ''}"
+				>
+					{item.label}
+				</a>
+			{/each}
+		{/each}
+	</nav>
+
+	<div data-testid="variety-widget-slot" class="mt-auto p-3"></div>
+</aside>