feat(invite): stub household invite page as onboarding Continue target

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

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

@@ -0,0 +1,7 @@
+<svelte:head>
+	<title>Mitglieder einladen — 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)]">A4 — Mitglieder einladen (coming soon)</p>
+</div>