refine: center toast notifications and silence plugin sync feedback

- Change toast position from top-right to top-center for better visibility
- Remove success notifications for plugin sync operations to reduce noise
- Keep error notifications to alert users of actual issues

src/components/ui/sonner.tsx

@@ -3,7 +3,7 @@ import { Toaster as SonnerToaster } from "sonner";
 export function Toaster() {
   return (
     <SonnerToaster
-      position="top-right"
+      position="top-center"
       richColors
       theme="system"
       toastOptions={{