Add a circular progress indicator and explicit phase labels

[PurpleSentinel]

Parent: #23

Summary

Use the round display to show session progress and clearly identify track, warning, overtime, and rest phases.

Acceptance criteria

• Keep the countdown as the primary visual element.
• Update progress no more often than needed for smooth, efficient rendering.
• Use labels or symbols so phase changes do not depend only on colour.
• Guarantee no clipping around the circular edge.
• Measure redraw cost and input responsiveness on hardware.

Roadmap group

Driver display and alerts