Fix #7575: Correct interface check for filer address function in admin UI

Problem:
User creation in Admin UI was failing with error:
'filer_etc: filer address function not configured'

Root Cause:
1. In admin_server.go, the code checked for incorrect interface method
   SetFilerClient(string, grpc.DialOption) instead of the actual
   SetFilerAddressFunc(func() pb.ServerAddress, grpc.DialOption)
2. The admin command was missing the filer_etc import, so the store
   was never registered

This interface mismatch prevented the filer address function from being
configured, causing user creation operations to fail in the Admin UI.

Note: This bug only affects the Admin UI. The S3 API and weed shell
commands (s3.configure) were unaffected as they use the correct interface
or bypass the credential manager entirely.

Solution:
- Added filer_etc import to weed/command/admin.go to register the store
- Fixed interface check in admin_server.go to use SetFilerAddressFunc
- Updated function call to properly configure filer address function
- Function now dynamically returns current active filer (HA-aware)
- Hoisted credentialManager assignment to reduce code duplication

Tests Added:
- Unit tests in weed/admin/dash/user_management_test.go
  * TestFilerAddressFunctionInterface - verifies correct interface
  * TestGenerateAccessKey - tests key generation
  * TestGenerateSecretKey - tests secret generation
  * TestGenerateAccountId - tests account ID generation

All tests pass and will run automatically in CI.

weed/admin/dash/admin_server.go

@@ -98,15 +98,13 @@ func NewAdminServer(masters string, templateFS http.FileSystem, dataDir string)
 		glog.Warningf("Failed to initialize credential manager: %v", err)
 		// Continue without credential manager - will fall back to legacy approach
 	} else {
+		server.credentialManager = credentialManager
+
 		// For stores that need filer address function, set them
 		if store := credentialManager.GetStore(); store != nil {
 			if filerFuncSetter, ok := store.(interface {
 				SetFilerAddressFunc(func() pb.ServerAddress, grpc.DialOption)
 			}); ok {
-			// We'll set the filer address function later when we discover filers
-			// For now, just store the credential manager
-			server.credentialManager = credentialManager
-
 				// Set up a goroutine to configure filer address function once we discover filers
 				go func() {
 					for {
@@ -123,11 +121,7 @@ func NewAdminServer(masters string, templateFS http.FileSystem, dataDir string)
 						time.Sleep(5 * time.Second)
 					}
 				}()
-			} else {
-				server.credentialManager = credentialManager
 			}
-		} else {
-			server.credentialManager = credentialManager
 		}
 	}
 

weed/command/admin.go

@@ -22,6 +22,7 @@ import (
 	"github.com/seaweedfs/seaweedfs/weed/admin"
 	"github.com/seaweedfs/seaweedfs/weed/admin/dash"
 	"github.com/seaweedfs/seaweedfs/weed/admin/handlers"
+	_ "github.com/seaweedfs/seaweedfs/weed/credential/filer_etc" // Register filer_etc credential store
 	"github.com/seaweedfs/seaweedfs/weed/pb"
 	"github.com/seaweedfs/seaweedfs/weed/security"
 	"github.com/seaweedfs/seaweedfs/weed/util"