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

.env.example

@@ -0,0 +1,9 @@
+# Lighting Controller UI Client Configuration
+
+# WebSocket URI for the control server
+# For local development (running UI on same machine as control server):
+CONTROL_SERVER_URI=ws://localhost:8765
+
+# For remote connection (running UI on desktop, control server on Pi):
+# Replace with your Raspberry Pi's IP address
+# CONTROL_SERVER_URI=ws://10.1.1.117:8765