Refactor security configuration: Remove external CrowdSec mode support

- Updated SecurityConfig model to only support 'local' or 'disabled' modes for CrowdSec.
- Modified related logic in the manager and services to reject external mode.
- Adjusted tests to validate the new restrictions on CrowdSec modes.
- Updated frontend components to remove references to external mode and provide appropriate user feedback.
- Enhanced documentation to reflect the removal of external CrowdSec mode support.

backend/internal/api/handlers/security_handler.go

@@ -57,9 +57,8 @@ func (h *SecurityHandler) GetStatus(c *gin.Context) {
 		}
 	}
 
-	// Treat external crowdsec mode as unsupported in this release. If configured as 'external',
-	// present it as disabled so the UI doesn't attempt to call out to an external agent.
-	if mode == "external" {
+	// Only allow 'local' as an enabled mode. Any other value should be treated as disabled.
+	if mode != "local" {
 		mode = "disabled"
 		apiURL = ""
 	}