feat(auth): add page title to signup screen

Sets <title>Konto erstellen — Mealprep</title> via svelte:head
for browser tab and accessibility.

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

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

@@ -3,6 +3,10 @@
 	import SignupForm from '$lib/auth/SignupForm.svelte';
 </script>
 
+<svelte:head>
+	<title>Konto erstellen — Mealprep</title>
+</svelte:head>
+
 <!-- Mobile: stacked, Desktop: side by side -->
 <div class="flex min-h-screen flex-col md:flex-row">
 	<BrandPanel />