Fix toast accept button: instant feedback + double-click guard

Toast buttons are static Sonner elements that can't bind React state,
so clicks had no visual feedback and allowed duplicate mutations.
Now: dismiss custom toast immediately, show loading toast, and block
concurrent clicks via a ref-based Set guard.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/notifications/NotificationToaster.tsx

@@ -14,16 +14,30 @@ export default function NotificationToaster() {
   const maxSeenIdRef = useRef(0);
   const initializedRef = useRef(false);
   const prevUnreadRef = useRef(0);
+  // Track in-flight request IDs so repeated clicks are blocked
+  const respondingRef = useRef<Set<number>>(new Set());
 
   const handleConnectionRespond = useCallback(
     async (requestId: number, action: 'accept' | 'reject', toastId: string | number) => {
+      // Guard against double-clicks (Sonner toasts are static, no disabled prop)
+      if (respondingRef.current.has(requestId)) return;
+      respondingRef.current.add(requestId);
+
+      // Immediately dismiss the custom toast and show a loading indicator
+      toast.dismiss(toastId);
+      const loadingId = toast.loading(
+        action === 'accept' ? 'Accepting connection…' : 'Declining request…',
+      );
+
       try {
         await respond({ requestId, action });
-        // onSuccess in useConnections already dismisses the custom toast and invalidates caches
+        toast.dismiss(loadingId);
         toast.success(action === 'accept' ? 'Connection accepted' : 'Request declined');
       } catch (err) {
-        toast.dismiss(toastId);
+        toast.dismiss(loadingId);
         toast.error(getErrorMessage(err, 'Failed to respond to request'));
+      } finally {
+        respondingRef.current.delete(requestId);
       }
     },
     [respond],