Fix connection mutation delays: make all onSuccess fire-and-forget

sendRequestMutation and cancelMutation were awaiting query invalidation
in onSuccess, which blocked mutateAsync until 3+ network refetches
completed (~1s+ delay). This caused noticeable lag on send/link/cancel
operations. Now all mutations use sync fire-and-forget invalidation,
matching respondMutation's pattern.

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

frontend/src/hooks/useConnections.ts

@@ -47,8 +47,9 @@ export function useConnections() {
       });
       return data;
     },
-    onSuccess: async () => {
-      await queryClient.invalidateQueries({ queryKey: ['connections'] });
+    onSuccess: () => {
+      // Fire-and-forget — don't block mutateAsync on query refetches
+      queryClient.invalidateQueries({ queryKey: ['connections'] });
     },
   });
 
@@ -72,11 +73,10 @@ export function useConnections() {
       const { data } = await api.put(`/connections/requests/${requestId}/cancel`);
       return data;
     },
-    onSuccess: async () => {
-      await Promise.all([
-        queryClient.invalidateQueries({ queryKey: ['connections'] }),
-        queryClient.invalidateQueries({ queryKey: ['notifications'] }),
-      ]);
+    onSuccess: () => {
+      // Fire-and-forget — don't block mutateAsync on query refetches
+      queryClient.invalidateQueries({ queryKey: ['connections'] });
+      queryClient.invalidateQueries({ queryKey: ['notifications'] });
     },
   });