Add operating modes and graphical G meter

[PurpleSentinel]

What changed

• add a five-second hold gesture and operating-mode menu
• preserve the existing Timer Mode workflow behind a memory-aware dispatcher
• add a responsive graphical G Mode with live vector, trail, peak marker, and high-visibility peak arc
• add persistent brightness settings and confirmed default restoration
• add safe IMU/touch fallback behavior and configuration migration
• update the README, user guide, upload instructions, and embedded version for v4.0.0

Why

The timer previously exposed only its track/rest workflow. This release adds selectable operating modes and makes the onboard IMU useful as a live graphical G meter while retaining the existing timer and its safety behavior.

The first G-meter implementation also used conservative filtering and a fixed post-render delay, which made the live marker lag. The final implementation responds by 60% on the first frame and accounts for LCD render time in its frame pacing. Single-pixel peak-arc markers were enlarged to filled bright-red dots for visibility on the physical display.

User impact

Users can hold the Ready or G Mode screen to switch modes, view live and maximum acceleration graphically, adjust display brightness, and restore defaults. Selections persist across restarts. Timer Mode remains available when the IMU is unavailable.

Validation

• python -m unittest discover -s tests -v — 116 tests passed
• python -m compileall -q .
• JSON configuration parse check
• git diff --check
• verified on the connected Waveshare RP2040-Touch-LCD-1.28
• measured G-meter update rate improved from approximately 5.3 FPS to 11.45 FPS before the final visibility-only marker adjustment

Closes #59