The readiness same-day grouping and the mood-streak anchor/walk both
sliced the UTC instant for their day key, mis-bucketing a late-evening
reading for a logger east or west of UTC. Thread an optional tz (default
Europe/Berlin) through both paths and key the buckets via userDayKey so
the day boundary follows the user's wall clock. Centralise the inline
day-in-ms literal in a shared time-constants module and use it here.
