feat(auth): add signup page with form action

Composes BrandPanel + SignupForm in responsive split layout.
Server action POSTs to /v1/auth/signup and redirects to
/household/setup on success.

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

frontend/src/routes/(public)/signup/+page.svelte

@@ -0,0 +1,14 @@
+<script lang="ts">
+	import BrandPanel from '$lib/auth/BrandPanel.svelte';
+	import SignupForm from '$lib/auth/SignupForm.svelte';
+</script>
+
+<!-- Mobile: stacked, Desktop: side by side -->
+<div class="flex min-h-screen flex-col md:flex-row">
+	<BrandPanel />
+	<div class="flex flex-1 flex-col justify-center px-[20px] py-[24px] md:px-[56px] md:py-[48px]">
+		<div class="max-w-[380px]">
+			<SignupForm />
+		</div>
+	</div>
+</div>