fix(onboarding): make HouseholdSetupForm heading responsive and use font-medium

text-[18px] md:text-[28px] matches auth form pattern.
font-medium (500) replaces font-semibold (600) per design system rules.

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

frontend/src/lib/onboarding/HouseholdSetupForm.svelte

@@ -38,7 +38,7 @@
 </script>
 
 <form method="POST" novalidate use:enhance onsubmit={handleSubmit}>
-	<h1 class="mb-[8px] font-[var(--font-display)] text-[24px] font-semibold">Haushalt benennen</h1>
+	<h1 class="mb-[8px] font-[var(--font-display)] text-[18px] font-medium md:text-[28px]">Haushalt benennen</h1>
 	<p class="mb-[24px] text-[14px] text-[var(--color-text-muted)]">
 		Gib deinem Haushalt einen Namen, damit du ihn leicht wiederfindest.
 	</p>