Add battery level, low-battery warnings, and power-saving behavior

[PurpleSentinel]

Parent: #23

Summary

Use the onboard battery ADC to present useful power status and reduce unexpected shutdowns.

Acceptance criteria

• Calibrate ADC readings for the supported board and battery chemistry.
• Show a stable battery estimate without distracting from the live timer.
• Provide configurable low and critical warnings.
• Reduce brightness or enter a safe low-power state when appropriate.
• Document accuracy limits and validate against measured voltage.

Roadmap group

Sensors, power, and hardware expansion