feat(invites): group picker in new-invite form

- load() fetches /api/groups in parallel with /api/invites; returns
  sorted groups array and groupsLoadError for partial failures
- create action forwards groupIds[] to POST /api/invites so invited
  users are placed in the selected groups on registration
- +page.svelte: group checkboxes via UserGroupsSection inside the form;
  amber warning banner when groups could not be loaded
- page.svelte.test.ts: groups checkboxes + warning banner tests
- page.server.test.ts: parallel fetch, sorting, error fallback,
  groupIds in POST body

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

frontend/src/routes/admin/invites/+page.svelte

@@ -2,7 +2,8 @@
 import { enhance } from '$app/forms';
 import { m } from '$lib/paraglide/messages.js';
 import { getErrorMessage } from '$lib/shared/errors';
-import type { InviteListItem } from './+page.server.ts';
+import UserGroupsSection from '$lib/user/UserGroupsSection.svelte';
+import type { InviteListItem, UserGroup } from './+page.server.ts';
 
 let {
 	data,
@@ -12,6 +13,8 @@ let {
 		invites: InviteListItem[];
 		status: string;
 		loadError: string | null;
+		groups: UserGroup[];
+		groupsLoadError: string | null;
 	};
 	form?: {
 		createError?: string;
@@ -253,6 +256,20 @@ function statusIcon(status: string) {
 							class="block w-full border border-line px-3 py-2 font-serif text-sm text-ink focus:outline-none focus-visible:ring-2 focus-visible:ring-focus-ring"
 						/>
 					</div>
+					<div class="sm:col-span-2">
+						<p class="mb-2 font-sans text-xs font-bold tracking-widest text-ink-3 uppercase">
+							{m.admin_new_invite_groups()}
+						</p>
+						{#if data.groupsLoadError}
+							<div
+								class="rounded-sm border border-amber-200 bg-amber-50 px-3 py-2 font-sans text-xs text-amber-700"
+							>
+								{m.admin_invite_groups_load_error()}
+							</div>
+						{:else}
+							<UserGroupsSection groups={data.groups} />
+						{/if}
+					</div>
 					{#if form?.createError}
 						<div class="font-sans text-xs font-medium text-red-600 sm:col-span-2">
 							{getErrorMessage(form.createError)}