Add local-day activity summaries derived from UTC hour observations using
the system timezone, with durable storage in a new local_days table.
The collector derives local-day read/write totals after UTC aggregation;
the TUI displays them with timezone, completeness state, and coverage.
Schema: bump SCHEMA_VERSION to 3, add local_days table (migration 2→3
is pure addition, idempotent, preserves newer-schema refusal).
