docs: clarify battery and thermal safety

[vku2018]

Summary

• add a prominent Fast Recovery section with the exact pmset off command
• clarify default battery safety behavior and the 30-second power-state refresh
• document that the app does not monitor temperature or replace macOS thermal protection
• restate that the optional sudoers helper remains limited to the two exact pmset commands

Validation

• git diff --check

Notes

• swift is not available in this Windows environment, so I did not run a Swift build. This change only updates README documentation.

Closes #2