feat(frontend): invite-based registration UI

- Add /register route with invite code prefill, password show/hide
- Add /login?registered=1 success banner
- Add /admin/invites page: list, create, revoke, copy link
- Add Einladungen nav section to admin sidebar (ADMIN_USER perm)
- Add invite error codes to errors.ts
- Add 48 i18n keys across de/en/es
- Update hooks.server.ts to allow public access to invite/register API

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

frontend/src/routes/login/+page.svelte

@@ -2,7 +2,10 @@
 import { m } from '$lib/paraglide/messages.js';
 import AuthHeader from '../AuthHeader.svelte';
 
-let { form }: { form?: { error?: string; success?: boolean } } = $props();
+let {
+	data,
+	form
+}: { data: { registered: boolean }; form?: { error?: string; success?: boolean } } = $props();
 </script>
 
 <svelte:head>
@@ -25,6 +28,16 @@ let { form }: { form?: { error?: string; success?: boolean } } = $props();
 
 			<!-- Card -->
 			<div class="rounded-sm border border-line bg-surface p-8 shadow-sm">
+				{#if data.registered}
+					<div
+						role="status"
+						aria-live="polite"
+						class="mb-5 rounded-sm border border-green-200 bg-green-50 px-4 py-3 font-sans text-xs font-medium text-green-800"
+					>
+						{m.login_registered_success()}
+					</div>
+				{/if}
+
 				<h1 class="mb-6 font-sans text-sm font-bold tracking-widest text-ink uppercase">
 					{m.login_heading()}
 				</h1>