Add accessible and colour-blind display themes

[PurpleSentinel]

Parent: #23

Summary

Offer high-contrast palettes and warning states that do not rely on colour alone.

Acceptance criteria

• Provide at least a default, high-contrast, and colour-blind-safe theme.
• Pair warning colours with text, symbols, or patterns.
• Validate contrast and readability on the physical LCD.
• Persist and preview the selected theme.
• Build on the warning-colour correction tracked by issue Expiry colours not great #2.

Roadmap group

Driver display and alerts