fix: Admin UI user creation fails before filer discovery (#7624)

The credential manager's filer address function was not configured quickly
enough after admin server startup, causing 'filer address function not
configured' errors when users tried to create users immediately.

Changes:
- Use exponential backoff (200ms -> 5s) instead of fixed 5s polling for
  faster filer discovery on startup
- Improve error messages to be more user-friendly and actionable

Fixes #7624

weed/admin/dash/admin_server.go

@@ -106,7 +106,10 @@ func NewAdminServer(masters string, templateFS http.FileSystem, dataDir string)
 				SetFilerAddressFunc(func() pb.ServerAddress, grpc.DialOption)
 			}); ok {
 				// Set up a goroutine to configure filer address function once we discover filers
+				// Use aggressive initial retries with exponential backoff for faster startup
 				go func() {
+					retryInterval := 200 * time.Millisecond // Start with fast retries
+					maxInterval := 5 * time.Second
 					for {
 						filerAddr := server.GetFilerAddress()
 						if filerAddr != "" {
@@ -114,11 +117,16 @@ func NewAdminServer(masters string, templateFS http.FileSystem, dataDir string)
 							filerFuncSetter.SetFilerAddressFunc(func() pb.ServerAddress {
 								return pb.ServerAddress(server.GetFilerAddress())
 							}, server.grpcDialOption)
-							glog.V(1).Infof("Set filer address function for credential manager: %s", filerAddr)
+							glog.V(0).Infof("Credential manager configured with filer: %s", filerAddr)
 							break
 						}
-						glog.V(1).Infof("Waiting for filer discovery for credential manager...")
-						time.Sleep(5 * time.Second)
+						glog.V(1).Infof("Waiting for filer discovery for credential manager (retry in %v)...", retryInterval)
+						time.Sleep(retryInterval)
+						// Exponential backoff up to maxInterval
+						retryInterval = retryInterval * 2
+						if retryInterval > maxInterval {
+							retryInterval = maxInterval
+						}
 					}
 				}()
 			}

weed/credential/filer_etc/filer_etc_store.go

@@ -58,7 +58,7 @@ func (store *FilerEtcStore) withFilerClient(fn func(client filer_pb.SeaweedFiler
 	store.mu.RLock()
 	if store.filerAddressFunc == nil {
 		store.mu.RUnlock()
-		return fmt.Errorf("filer_etc: filer address function not configured")
+		return fmt.Errorf("filer_etc: filer not yet available - please wait for filer discovery to complete and try again")
 	}
 
 	filerAddress := store.filerAddressFunc()
@@ -66,7 +66,7 @@ func (store *FilerEtcStore) withFilerClient(fn func(client filer_pb.SeaweedFiler
 	store.mu.RUnlock()
 	
 	if filerAddress == "" {
-		return fmt.Errorf("filer_etc: filer address is empty")
+		return fmt.Errorf("filer_etc: no filer discovered yet - please ensure a filer is running and accessible")
 	}
 
 	// Use the pb.WithGrpcFilerClient helper similar to existing code