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/api/client.ts

@@ -64,15 +64,27 @@ client.interceptors.response.use(
       }
     }
 
-    // Extract error message from response
+    // Extract error message — detail can be a string or a structured object
+    const rawDetail = error.response?.data?.detail;
     const message =
-      error.response?.data?.detail ||
-      error.message ||
-      "An unexpected error occurred";
+      typeof rawDetail === "string"
+        ? rawDetail
+        : typeof rawDetail === "object" && rawDetail !== null && (rawDetail as { message?: string }).message
+          ? (rawDetail as { message: string }).message
+          : error.message || "An unexpected error occurred";
 
-    const enhancedError = new Error(message);
-    (enhancedError as Error & { status?: number }).status = status;
-    (enhancedError as Error & { code?: string }).code = error.response?.data?.code;
+    const enhancedError = new Error(message) as Error & {
+      status?: number;
+      code?: string;
+      detail?: unknown;
+    };
+    enhancedError.status = status;
+    // Preserve the full detail object so consumers can inspect it (e.g. for modal flows)
+    enhancedError.detail = rawDetail;
+    enhancedError.code =
+      typeof rawDetail === "object" && rawDetail !== null
+        ? (rawDetail as { code?: string }).code
+        : error.response?.data?.code;
 
     return Promise.reject(enhancedError);
   },