Make warning thresholds and overtime behavior configurable

[PurpleSentinel]

Parent: #23

Summary

Allow users to choose when warnings occur and what the timer does after scheduled time expires.

Acceptance criteria

• Configure at least two warning thresholds by percentage or remaining time.
• Validate ordering and keep safe defaults matching current behavior.
• Offer continue-counting, count-up overtime, auto-finish, and grace-period policies.
• Show the selected policy in configuration and documentation.
• Coordinate visual colour semantics with existing issue Expiry colours not great #2.

Roadmap group

Session workflow and timing