Add Ready-screen motion-based Auto-Dim

[PurpleSentinel]

What changed

• add a persistent On/Off Auto-Dim choice under Settings
• dim only the Timer Ready screen after 10 seconds without detected motion
• restore the saved display brightness immediately on motion or when leaving Ready
• configure the dim level with validated AUTO_DIM_PERCENT in params.json
• retain normal Timer operation and brightness if the QMI8658 is unavailable
• document configuration, operation, migration, and deployment

Why

The Ready screen can remain powered for extended periods. Motion-based dimming reduces unnecessary backlight output while preserving immediate readability and leaving every active or interactive screen unaffected.

Validation

• python3 -m unittest discover -s tests -v — 185 tests pass
• Python compilation and diff whitespace checks pass
• connected Waveshare hardware dimmed at 10.043 seconds
• measured backlight PWM at exactly 25% (6250 / 24999)
• verified immediate motion wake, Ready exit restoration, settings persistence, and clean full-firmware boot

Closes #72