fix(docker): use API version negotiation instead of hardcoded 1.41 (#636)

The default Docker API version was hardcoded to 1.41, which fails on
newer Docker daemons that require at least API version 1.44.

Change the default to empty string so the Docker client uses the
existing WithAPIVersionNegotiation() option to auto-negotiate a
compatible version with the daemon.

apps/backend/internal/common/config/config.go

@@ -263,7 +263,7 @@ func setDefaults(v *viper.Viper) {
 	// Docker defaults — platform-aware host and volume path
 	v.SetDefault("docker.enabled", true) // Docker runtime enabled by default if Docker is available
 	v.SetDefault("docker.host", DefaultDockerHost())
-	v.SetDefault("docker.apiVersion", "1.41")
+	v.SetDefault("docker.apiVersion", "") // Empty = auto-negotiate with daemon
 	v.SetDefault("docker.tlsVerify", false)
 	v.SetDefault("docker.defaultNetwork", "kandev-network")
 	v.SetDefault("docker.volumeBasePath", defaultDockerVolumePath())