feat(invites): show empty state when no groups exist in invite form

When groups load successfully but the list is empty, render a quiet
"Keine Gruppen vorhanden." message rather than a blank section that
leaves users uncertain whether groups failed to load.

Adds admin_new_invite_no_groups i18n key to de/en/es.

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

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

@@ -267,6 +267,8 @@ function statusIcon(status: string) {
 							>
 								{m.admin_invite_groups_load_error()}
 							</div>
+						{:else if data.groups.length === 0}
+							<p class="font-sans text-xs text-ink-3 italic">{m.admin_new_invite_no_groups()}</p>
 						{:else}
 							<UserGroupsSection groups={data.groups} />
 						{/if}

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

@@ -368,5 +368,7 @@ describe('admin/invites page', () => {
 
 		expect(document.querySelectorAll('input[name="groupIds"]')).toHaveLength(0);
 		expect(document.querySelector('.bg-amber-50')).toBeNull();
+		// empty-state message visible — "Keine Gruppen vorhanden." in de locale
+		await expect.element(page.getByText(/keine gruppen/i)).toBeVisible();
 	});
 });