feat(auth): add use:enhance and server error display to signup form

SignupForm now uses use:enhance for progressive enhancement.
Accepts form prop for server-side error display. Shows general
form errors in a banner and field-specific errors inline.

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

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

@@ -1,6 +1,8 @@
 <script lang="ts">
 	import BrandPanel from '$lib/auth/BrandPanel.svelte';
 	import SignupForm from '$lib/auth/SignupForm.svelte';
+
+	let { form } = $props();
 </script>
 
 <svelte:head>
@@ -12,7 +14,7 @@
 	<BrandPanel />
 	<div class="flex flex-1 flex-col items-start justify-center px-[20px] py-[24px] md:items-center md:px-[56px] md:py-[48px]">
 		<div class="w-full max-w-[380px]">
-			<SignupForm />
+			<SignupForm {form} />
 		</div>
 	</div>
 </div>