fix(onboarding): make HouseholdSetupForm subtitle responsive (12px mobile, 14px desktop)

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

frontend/src/lib/onboarding/HouseholdSetupForm.svelte

@@ -39,7 +39,7 @@
 
 <form method="POST" novalidate use:enhance onsubmit={handleSubmit}>
 	<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)]">
+	<p class="mb-[24px] text-[12px] text-[var(--color-text-muted)] md:text-[14px]">
 		Gib deinem Haushalt einen Namen, damit du ihn leicht wiederfindest.
 	</p>