Add battery level reporting via ADC voltage divider

[xishell]

Description

Read the battery voltage through the ADC pin (GPIO 5) connected to a 100 kOhm voltage divider. Convert the raw ADC reading to a battery percentage (0-100) and include it in the ESP-NOW status payload.

Acceptance Criteria

• ADC reads battery voltage on GPIO 5
• Raw ADC value is converted to a 0-100 percentage
• Battery percentage is included in the transmitted status payload