Add Ready-screen battery indicator

[PurpleSentinel]

Summary

• add a standard proportionally filled battery icon above the Ready heading
• overlay a high-contrast lightning bolt when RP2040 VBUS is present
• estimate battery-only charge from smoothed GP29 readings and a bounded Li-ion curve
• retain the last valid battery estimate while externally powered, with a documented startup fallback
• update the user guide, README, and device installation command

Why

The Ready screen did not expose any power status. The Waveshare board provides a divided VSYS reading on GP29 and an RP2040 VBUS status bit, but USB power masks the isolated battery voltage. The implementation therefore reports battery-only voltage honestly and treats the initial full fill under USB as powered status rather than an exact cell measurement.

User impact

Drivers can see approximate remaining charge before starting a session and immediately recognize external power from the charging bolt. The 35×16 px icon is centered at the top of the circular display with a 12 px gap before Ready, preserving all existing settings and instructions.

Validation

• python -m unittest discover -s tests -v — 82 tests passed
• syntax and whitespace checks passed
• verified VBUS detection and GP29 readings on the supported Waveshare RP2040 board
• installed on hardware and visually inspected the framebuffer for icon clarity, charging-bolt contrast, circular fit, and title clearance

Closes#55