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.server.ts

@@ -1,6 +1,11 @@
 import { fail, redirect, type Actions } from '@sveltejs/kit';
 import { env } from '$env/dynamic/private';
 import { getErrorMessage } from '$lib/errors';
+import type { PageServerLoad } from './$types';
+
+export const load: PageServerLoad = ({ url }) => {
+	return { registered: url.searchParams.get('registered') === '1' };
+};
 
 export const actions = {
 	login: async ({ request, cookies, fetch }) => {