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/admin/invites/+page.server.ts

@@ -0,0 +1,88 @@
+import { fail } from '@sveltejs/kit';
+import { env } from '$env/dynamic/private';
+import { parseBackendError } from '$lib/errors';
+import type { Actions, PageServerLoad } from './$types';
+
+export interface InviteListItem {
+	id: string;
+	code: string;
+	displayCode: string;
+	label?: string;
+	useCount: number;
+	maxUses?: number;
+	expiresAt?: string;
+	revoked: boolean;
+	status: string;
+	createdAt: string;
+	shareableUrl: string;
+}
+
+export const load: PageServerLoad = async ({ url, fetch }) => {
+	const status = url.searchParams.get('status') ?? 'active';
+	const apiUrl = env.API_INTERNAL_URL || 'http://localhost:8080';
+	const res = await fetch(`${apiUrl}/api/invites?status=${encodeURIComponent(status)}`);
+
+	if (!res.ok) {
+		const backendError = await parseBackendError(res);
+		return {
+			invites: [] as InviteListItem[],
+			status,
+			loadError: backendError?.code ?? 'INTERNAL_ERROR'
+		};
+	}
+
+	const invites: InviteListItem[] = await res.json();
+	return { invites, status, loadError: null };
+};
+
+export const actions = {
+	create: async ({ request, fetch }) => {
+		const formData = await request.formData();
+		const label = (formData.get('label') as string) || undefined;
+		const maxUsesRaw = formData.get('maxUses') as string;
+		const maxUses = maxUsesRaw ? parseInt(maxUsesRaw, 10) : undefined;
+		const prefillFirstName = (formData.get('prefillFirstName') as string) || undefined;
+		const prefillLastName = (formData.get('prefillLastName') as string) || undefined;
+		const prefillEmail = (formData.get('prefillEmail') as string) || undefined;
+		const expiresAt = (formData.get('expiresAt') as string) || undefined;
+
+		const apiUrl = env.API_INTERNAL_URL || 'http://localhost:8080';
+		const res = await fetch(`${apiUrl}/api/invites`, {
+			method: 'POST',
+			headers: { 'Content-Type': 'application/json' },
+			body: JSON.stringify({
+				label,
+				maxUses,
+				prefillFirstName,
+				prefillLastName,
+				prefillEmail,
+				expiresAt
+			})
+		});
+
+		if (!res.ok) {
+			const backendError = await parseBackendError(res);
+			return fail(res.status, { createError: backendError?.code ?? 'INTERNAL_ERROR' });
+		}
+
+		const created: InviteListItem = await res.json();
+		return { created };
+	},
+
+	revoke: async ({ request, fetch }) => {
+		const formData = await request.formData();
+		const id = formData.get('id') as string;
+
+		const apiUrl = env.API_INTERNAL_URL || 'http://localhost:8080';
+		const res = await fetch(`${apiUrl}/api/invites/${encodeURIComponent(id)}`, {
+			method: 'DELETE'
+		});
+
+		if (!res.ok) {
+			const backendError = await parseBackendError(res);
+			return fail(res.status, { revokeError: backendError?.code ?? 'INTERNAL_ERROR' });
+		}
+
+		return { revoked: id };
+	}
+} satisfies Actions;