fix(auth): center signup form on wide desktop screens

Form container now horizontally centered on md+ viewports,
left-aligned on mobile for full-width usage.

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

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

@@ -10,8 +10,8 @@
 <!-- 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]">
+	<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 />
 		</div>
 	</div>