fix(campaigns): hide direct remove-test button on active campaigns

Direct test removal is now gated to draft-only at the service layer
(active campaigns must go through a modification request that a
manager approves). The trash-icon button on the tests table still
showed for active campaigns and would always fail with a 400 —
restrict it to draft, matching the backend gate.

frontend/src/pages/CampaignDetailPage.tsx

@@ -734,7 +734,7 @@ export default function CampaignDetailPage() {
                           >
                             <Clock className="h-4 w-4" />
                           </button>
-                          {canManage && (campaign.status === "draft" || campaign.status === "active") && (
+                          {canManage && campaign.status === "draft" && (
                             <button
                               onClick={() => removeMutation.mutate(ct.id)}
                               disabled={removeMutation.isPending}