Add .env file support for UI client configuration

- Use python-dotenv to load environment variables
- Add CONTROL_SERVER_URI environment variable for WebSocket connection
- Create .env.example with configuration examples
- Update Pipfile to include python-dotenv dependency
- Allows easy configuration for running UI on desktop pointing to Pi

Pipfile

@@ -13,6 +13,7 @@ python-rtmidi = "*"
 pyaudio = "*"
 aubio = "*"
 websocket-client = "*"
+python-dotenv = "*"
 
 [dev-packages]