fix: surface remaining silent mutation failures

Four more mutations had no error feedback: equipment condition change,
task completion, and notification mark-as-read all failed silently.
More importantly, the offline sync badge computed a result.failed
count from syncPendingInspections() but never displayed it - an entry
rejected by the server (e.g. a validation rule) would sit in the
offline queue indefinitely with zero indication to the beekeeper why
it never syncs. Both the failed-count and outright sync-mutation
rejection now surface a toast.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

ApiCore-Frontend/src/components/notification-bell.tsx

@@ -3,6 +3,7 @@
 import Link from "next/link";
 import { useQuery, useMutation, useQueryClient } from "@tanstack/react-query";
 import { Bell } from "lucide-react";
+import { toast } from "sonner";
 import { fetchNotifications, markNotificationRead } from "@/lib/notifications/api";
 import { Notification, NotificationPriority } from "@/lib/notifications/types";
 import { Badge, badgeVariants } from "@/components/ui/badge";
@@ -41,6 +42,7 @@ export function NotificationBell() {
     onSuccess: () => {
       void queryClient.invalidateQueries({ queryKey: ["notifications"] });
     },
+    onError: () => toast.error("Benachrichtigung konnte nicht als gelesen markiert werden"),
   });
 
   const unreadCount = notifications?.filter((notification) => !notification.read).length ?? 0;

ApiCore-Frontend/src/components/offline-sync-badge.tsx

@@ -26,7 +26,14 @@ export function OfflineSyncBadge() {
       if (result.synced > 0) {
         toast.success(`${result.synced} Eintrag/Einträge synchronisiert`);
       }
+      if (result.failed > 0) {
+        toast.error(
+          `${result.failed} Eintrag/Einträge konnten nicht synchronisiert werden und bleiben in der Warteschlange`,
+        );
+      }
     },
+    onError: () =>
+      toast.error("Synchronisierung fehlgeschlagen - wird beim nächsten Versuch erneut versucht"),
   });
 
   useEffect(() => {