- Add ErrFutureDay sentinel error
- CreateInterval: rejects if startDayKey > todayKey (400)
- Update: rejects if new start_time moves entry to a future day (400)
- Handler maps ErrFutureDay → 400 Bad Request for both endpoints
- Add TestCreateIntervalRejectsFutureDay
- Add TestUpdateRejectsMoveToFutureDay
- UI already gates this via dayCapabilities (canAddInterval=false,
canEditEntries=false for future days), but server now enforces it too