fix speaker volume control and auth

- Rename USERNAME/PASSWORD to UNIFI_USERNAME/UNIFI_PASSWORD to avoid
  conflict with the Windows USERNAME system environment variable
- Set both speakerSettings.volume and speakerSettings.speakerVolume
  in the PATCH request
- Log volume before and after the change to verify it took effect

.env.example

@@ -1,2 +1,4 @@
 HOST=192.168.1.1
 API_KEY=
+UNIFI_USERNAME=
+UNIFI_PASSWORD=