fix(campaigns): fix start_date modal — interceptor was losing structured detail

client.ts: when FastAPI detail is an object, extract .message for the error
string and preserve the full detail on enhancedError.detail so consumers
can inspect structured error payloads (e.g. 409 start_date_in_future).

CampaignDetailPage: use enhancedErr.status (not response.status) and
enhancedErr.detail (not response.data.detail) to detect 409 and show
the confirmation modal instead of the toast.

frontend/src/pages/CampaignDetailPage.tsx

@@ -98,29 +98,23 @@ export default function CampaignDetailPage() {
       showToast("Campaign activated", "success");
     },
     onError: (err: unknown) => {
-      // FastAPI wraps error bodies as: { detail: string | object }
-      type AxiosLike = {
-        response?: {
-          status?: number;
-          data?: { detail?: { code?: string; message?: string } | string };
-        };
+      // The Axios interceptor (client.ts) transforms errors into enhanced Error objects
+      // with .status (HTTP status), .detail (raw FastAPI detail), and .message (readable string)
+      type EnhancedError = Error & {
+        status?: number;
+        detail?: { code?: string; message?: string } | string;
       };
-      const axiosErr = err as AxiosLike;
-      const status = axiosErr?.response?.status;
-      const detail = axiosErr?.response?.data?.detail;
+      const e = err as EnhancedError;
 
-      if (status === 409 && detail && typeof detail === "object" && detail.message) {
-        // Future start_date → show confirmation modal
-        setStartDateWarning(detail.message);
+      if (e.status === 409) {
+        // Future start_date — show confirmation modal using the message from detail
+        const warningMsg =
+          typeof e.detail === "object" && e.detail?.message
+            ? e.detail.message
+            : e.message;
+        setStartDateWarning(warningMsg);
       } else {
-        // 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");
+        showToast(e.message || "Failed to activate campaign", "error");
       }
     },
   });