fix(campaigns): correct Axios error parsing in activateMutation

FastAPI wraps error bodies as {detail: string | object}, not at the
top level. Was reading data.message instead of data.detail.message,
causing [object Object] in the toast for all non-409 errors.

Now correctly extracts:
- 409 with object detail -> start_date warning modal
- Other errors with string detail -> readable toast message
- Other errors with object detail -> detail.message in toast

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

frontend/src/pages/CampaignDetailPage.tsx

@@ -98,12 +98,29 @@ export default function CampaignDetailPage() {
       showToast("Campaign activated", "success");
     },
     onError: (err: unknown) => {
-      // 409 = future start_date warning → show confirmation modal instead of toast
-      const axiosErr = err as { response?: { status?: number; data?: { message?: string; start_date?: string } } };
-      if (axiosErr?.response?.status === 409 && axiosErr.response.data?.message) {
-        setStartDateWarning(axiosErr.response.data.message);
+      // FastAPI wraps error bodies as: { detail: string | object }
+      type AxiosLike = {
+        response?: {
+          status?: number;
+          data?: { detail?: { code?: string; message?: string } | string };
+        };
+      };
+      const axiosErr = err as AxiosLike;
+      const status = axiosErr?.response?.status;
+      const detail = axiosErr?.response?.data?.detail;
+
+      if (status === 409 && detail && typeof detail === "object" && detail.message) {
+        // Future start_date → show confirmation modal
+        setStartDateWarning(detail.message);
       } else {
-        showToast((err as Error).message, "error");
+        // Any other error → extract readable message from FastAPI detail
+        const msg =
+          typeof detail === "string"
+            ? detail
+            : typeof detail === "object" && detail?.message
+              ? detail.message
+              : "Failed to activate campaign";
+        showToast(msg, "error");
       }
     },
   });