feat(relay): support configurable frps webui port and fix node detail integration

- Implement configurable FRPS WebUI switch and custom port setting (relay_frps_web_ui_port) in system configs.
- Integrate settings into Relay Node detail manage page instead of global settings.
- Dynamically query server version to select matching Docker image tag for Relay installation.
- Clean up legacy code and fix backend linter/test warnings.

pkg/protocol/server.go

@@ -43,6 +43,7 @@ type RelayConfig struct {
 	AuthToken        string `json:"auth_token"`
 	LogLevel         string `json:"log_level"`
 	WebServerEnabled bool   `json:"web_server_enabled"`
+	WebServerPort    int    `json:"web_server_port"`
 }
 
 // RelaySettings holds relay runtime settings.