Save Shutdown based on UPS_Lite.

**Is your feature request related to a problem? Please describe.**
When the battery is no longer able to support the Raspberry it falls flat on his face end the raspberry is just killed. We all know that losing data is just a waiting game.

**Describe the solution you'd like**
When the UPS_Lite plugin hits x% (based on config.yml) perform a save shutdown and update the screen so the user knows what's happened to our little friend.

Start by locating the UPS_Lite plugin and its config.yml settings, then trace how battery thresholds and screen updates are handled. Confirm the expected shutdown path and test that reaching the configured percentage produces a visible status update and a clean shutdown.

Written by the indexing model from the issue text.