Support custom track and rest durations

[PurpleSentinel]

Parent: #23

Summary

Allow durations outside the fixed minute preset list, including practical minute-and-second values.

Acceptance criteria

• Provide a touch-friendly editor that fits the round display.
• Validate minimum and maximum values and reject zero or unsafe durations.
• Persist custom track and rest values across reboot.
• Keep the existing presets available as shortcuts.
• Add host tests for editing, persistence, formatting, and boundary values.

Roadmap group

Session workflow and timing