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.test.ts

@@ -7,12 +7,15 @@ afterEach(cleanup);
 
 const makeInvite = (overrides: Record<string, unknown> = {}) => ({
 	id: 'i-1',
+	code: 'XYZ1234567',
 	displayCode: 'XYZ-1234',
 	label: 'Familie',
 	useCount: 0,
 	maxUses: 5,
 	expiresAt: '2027-01-01T00:00:00Z',
+	revoked: false,
 	status: 'active' as string,
+	createdAt: '2025-01-01T00:00:00Z',
 	shareableUrl: 'http://example.com/i/i-1',
 	...overrides
 });
@@ -22,11 +25,15 @@ const baseData = (
 		invites: ReturnType<typeof makeInvite>[];
 		status: string;
 		loadError: string | null;
+		groups: { id: string; name: string; permissions: string[] }[];
+		groupsLoadError: string | null;
 	}> = {}
 ) => ({
 	invites: [],
 	status: 'active',
 	loadError: null,
+	groups: [],
+	groupsLoadError: null,
 	...overrides
 });
 
@@ -253,4 +260,43 @@ describe('admin/invites page', () => {
 		const banner = document.querySelector('.bg-red-50');
 		expect(banner).not.toBeNull();
 	});
+
+	// ─── groups section ───────────────────────────────────────────────────────
+
+	it('shows a groups-load warning banner when data.groupsLoadError is set', async () => {
+		render(AdminInvitesPage, {
+			props: { data: { ...baseData(), groups: [], groupsLoadError: 'INTERNAL_ERROR' } }
+		});
+
+		await page
+			.getByRole('button', { name: /neue einladung/i })
+			.first()
+			.click();
+
+		const banner = document.querySelector('.bg-amber-50');
+		expect(banner).not.toBeNull();
+	});
+
+	it('renders group checkboxes inside the new-invite form when groups are provided', async () => {
+		render(AdminInvitesPage, {
+			props: {
+				data: {
+					...baseData(),
+					groups: [
+						{ id: 'g-1', name: 'Administratoren', permissions: ['ADMIN'] },
+						{ id: 'g-2', name: 'Familie', permissions: ['READ_ALL'] }
+					],
+					groupsLoadError: null
+				}
+			}
+		});
+
+		await page
+			.getByRole('button', { name: /neue einladung/i })
+			.first()
+			.click();
+
+		await expect.element(page.getByRole('checkbox', { name: 'Administratoren' })).toBeVisible();
+		await expect.element(page.getByRole('checkbox', { name: 'Familie' })).toBeVisible();
+	});
 });