feat(register): redesign register page to match spec

Replaces the minimal login-style form with the full spec design:
hero section (eyebrow, headline, subtext), three labelled form sections,
2-column name grid, confirm-password field with client-side match hints,
password strength indicator, notification checkbox card, loading state on
submit, and "already have an account?" footer link.

Backend: adds notifyOnMention to RegisterRequest and wires both
notifyOnMention and notifyOnReply via updateNotificationPreferences on
invite redemption.

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

frontend/src/routes/AuthHeader.svelte

@@ -5,7 +5,7 @@ import LanguageSwitcher from '$lib/components/LanguageSwitcher.svelte';
 <header class="bg-header">
 	<div class="h-1 bg-accent"></div>
 	<div class="mx-auto max-w-7xl px-4 sm:px-6 lg:px-8">
-		<div class="flex h-12 items-center justify-between">
+		<div class="flex h-16 items-center justify-between">
 			<a href="/" class="flex items-center" aria-label="Familienarchiv">
 				<span class="font-sans text-sm font-bold tracking-widest text-white uppercase"
 					>Familienarchiv</span