feat(onboarding): add A2 household setup page with responsive progress sidebar layout

Desktop: 300px ProgressSidebar (step 1 active) + flex form area.
Mobile: "Schritt 1 von 3" eyebrow + HouseholdSetupForm.
Also stubs /household/staples as redirect target for A3.

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

frontend/src/routes/household/staples/+page.svelte

@@ -0,0 +1,7 @@
+<svelte:head>
+	<title>Vorräte einrichten — Mealplan</title>
+</svelte:head>
+
+<div class="flex min-h-screen items-center justify-center bg-[var(--color-page)]">
+	<p class="text-[var(--color-text-muted)]">A3 — Vorräte einrichten (coming soon)</p>
+</div>