fix(invites): add role="alert" to groups-load-error banner

Screen readers now announce the amber warning when it appears after
the form expands, without requiring the user to navigate to it.

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

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

@@ -262,6 +262,7 @@ function statusIcon(status: string) {
 						</p>
 						{#if data.groupsLoadError}
 							<div
+								role="alert"
 								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()}