Add manual brightness control and night mode

[PurpleSentinel]

Parent: #23

Summary

Let users reduce glare, save power, and recall an appropriate display brightness.

Acceptance criteria

• Provide several safe brightness levels including a night setting.
• Preview brightness while editing and persist the selection.
• Ensure warnings remain legible at every supported level.
• Define startup and hardware-error brightness behavior.
• Add tests around configuration validation and PWM mapping.

Roadmap group

Driver display and alerts