Add a configurable pre-start countdown sequence

[PurpleSentinel]

Parent: #23

Summary

Provide an optional staged countdown before normal timing begins when Launch Mode is disabled.

Acceptance criteria

• Support immediate start, fixed countdown, and randomized start-window modes.
• Make the active mode clear before the user starts.
• Allow cancellation throughout the sequence.
• Use monotonic timing and avoid blocking touch polling.
• Test countdown boundaries and randomized ranges deterministically.

Roadmap group

Session workflow and timing