fix(invites): i18n legend + touch target in UserGroupsSection

- legend uses m.admin_new_invite_groups() instead of hardcoded "Gruppen"
  so screen readers announce the correct string in en/es locales
- label gets min-h-[44px] for WCAG 2.2 touch target compliance
- add test asserting fieldset accessible name comes from i18n key
- add test documenting empty-groups-no-error renders no checkboxes/banner

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

frontend/src/routes/admin/invites/page.svelte.test.ts

@@ -334,4 +334,39 @@ describe('admin/invites page', () => {
 		const alert = document.querySelector('[role="alert"]');
 		expect(alert).not.toBeNull();
 	});
+
+	it('checkbox group fieldset has accessible name from i18n key (not hardcoded German)', async () => {
+		render(AdminInvitesPage, {
+			props: {
+				data: {
+					...baseData(),
+					groups: [{ id: 'g-1', name: 'Familie', permissions: ['READ_ALL'] }],
+					groupsLoadError: null
+				}
+			}
+		});
+
+		await page
+			.getByRole('button', { name: /neue einladung/i })
+			.first()
+			.click();
+
+		// m.admin_new_invite_groups() returns "Gruppen (optional)" in de locale
+		// The hardcoded legend "Gruppen" would not match this accessible name
+		await expect.element(page.getByRole('group', { name: 'Gruppen (optional)' })).toBeVisible();
+	});
+
+	it('shows no checkboxes and no warning when groups list is empty and no error', async () => {
+		render(AdminInvitesPage, {
+			props: { data: { ...baseData(), groups: [], groupsLoadError: null } }
+		});
+
+		await page
+			.getByRole('button', { name: /neue einladung/i })
+			.first()
+			.click();
+
+		expect(document.querySelectorAll('input[name="groupIds"]')).toHaveLength(0);
+		expect(document.querySelector('.bg-amber-50')).toBeNull();
+	});
 });