Adds a sick-day exemption flow with debt tracking so workout enforcement
can be skipped on declared sick days while preserving phone-verification
and shutdown invariants.
- New _sick_tracker module persists sick_history.json (days, debt, commitments).
- New _sick_dialog integrates declaration into the lock UI flow.
- _ui_flows.py and screen_lock.py consult tracker before enforcing workouts.
- gitignore sick_history.json (runtime state, like sick_day_state.json).
- 304 tests pass; 100% branch coverage on every screen_locker file.
