feat(onboarding): add HouseholdSetupForm component with disabled-until-valid continue button

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

frontend/src/lib/onboarding/HouseholdSetupForm.svelte

@@ -0,0 +1,80 @@
+<script lang="ts">
+	import { enhance } from '$app/forms';
+
+	type FormResult = {
+		errors?: Record<string, string>;
+		name?: string;
+	} | null;
+
+	let { form = null }: { form?: FormResult } = $props();
+
+	let name = $state('');
+	let touched = $state(false);
+	let submitAttempted = $state(false);
+	let formError = $state('');
+
+	const isDisabled = $derived(name.trim().length === 0);
+	const error = $derived(
+		(touched || submitAttempted) && name.trim() === '' ? 'Haushaltsname ist erforderlich' : ''
+	);
+
+	$effect(() => {
+		if (form?.errors) {
+			formError = form.errors.form ?? '';
+			name = form?.name ?? '';
+		}
+	});
+
+	function handleSubmit(event: SubmitEvent) {
+		submitAttempted = true;
+		if (name.trim() === '') {
+			event.preventDefault();
+		}
+	}
+
+	function handleInput() {
+		touched = true;
+	}
+</script>
+
+<form method="POST" novalidate use:enhance onsubmit={handleSubmit}>
+	<h1 class="mb-[8px] font-['var(--font-display)'] text-[24px] font-semibold">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>
+
+	<div class="mb-[16px]">
+		<label for="name" class="mb-[6px] block text-[14px] font-medium">Haushaltsname</label>
+		<input
+			type="text"
+			id="name"
+			name="name"
+			placeholder="z.B. Familie Müller"
+			autocomplete="organization"
+			bind:value={name}
+			oninput={handleInput}
+			class="w-full rounded-[var(--radius-md)] border bg-[var(--color-page)] px-[12px] py-[10px] text-[14px] outline-none focus:ring-2 focus:ring-[var(--green-dark)] {error
+				? 'border-[var(--color-error)]'
+				: 'border-[var(--color-border)]'}"
+		/>
+		{#if error}
+			<p class="mt-1 text-[12px] text-[var(--color-error)]">{error}</p>
+		{/if}
+	</div>
+
+	{#if formError}
+		<p
+			class="mb-[16px] rounded-[var(--radius-md)] bg-[color-mix(in_srgb,var(--color-error),transparent_90%)] px-[12px] py-[10px] text-[12px] text-[var(--color-error)]"
+		>
+			{formError}
+		</p>
+	{/if}
+
+	<button
+		type="submit"
+		disabled={isDisabled}
+		class="w-full cursor-pointer rounded-[var(--radius-md)] bg-[var(--green-dark)] px-[24px] py-[12px] text-[14px] font-medium text-white disabled:cursor-not-allowed disabled:opacity-50"
+	>
+		Weiter → Vorräte einrichten
+	</button>
+</form>