Add an on-device self-test and diagnostic screen

[PurpleSentinel]

Parent: #23

Summary

Make it easy to confirm display, touch, IMU, flash, battery ADC, font assets, and configuration health.

Acceptance criteria

• Expose diagnostics from a deliberate Ready-screen action.
• Report pass, degraded, or failed status with actionable details.
• Exercise touch and display safely without entering a session.
• Provide a machine-readable copy through USB when available.
• Test diagnostic aggregation with mocked peripheral failures.

Roadmap group

Data, configuration, and maintenance