fix(demo): allow Manager to seed demo data + surface seed errors

The dashboard demo-data banner is shown to Owner and Manager, but the /demo/seed endpoint required strictly Owner, so a Manager clicking it got a silent 403 (the banner had no error handler) — appearing as 'nothing happens, no tables or items'. The endpoint now allows Owner or Manager, and the banner shows the error on failure.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/dashboard/src/components/demo/demo-data-banner.tsx

@@ -3,7 +3,8 @@
 import { useState } from "react";
 import { useMutation, useQueryClient } from "@tanstack/react-query";
 import { Sparkles, Loader2 } from "lucide-react";
-import { apiPost } from "@/lib/api/client";
+import { ApiClientError, apiPost } from "@/lib/api/client";
+import { notify } from "@/lib/notify";
 import { useAuthStore } from "@/lib/stores/auth.store";
 import { Button } from "@/components/ui/button";
 import { cn } from "@/lib/utils";
@@ -39,6 +40,13 @@ export function DemoDataBanner({ invalidateKeys, className }: Props) {
         qc.invalidateQueries({ queryKey: key });
       }
     },
+    onError: (err) => {
+      notify.error(
+        err instanceof ApiClientError
+          ? err.message
+          : "افزودن داده‌های نمونه ناموفق بود. دوباره تلاش کنید."
+      );
+    },
   });
 
   if (!cafeId || (role !== "Owner" && role !== "Manager")) return null;