getPresetLookbackStart subtracted (N-1) * 24h from the current instant
and then rounded to local midnight. Across a DST offset change that
subtraction can land on the adjacent date, so the 7d/14d/30d presets
included one extra local day after spring-forward and dropped one after
fall-back (e.g. America/New_York, Europe/Berlin). The shifted range fed
the usage summary, model/provider stats and request-log queries.
Build the start date from today's local year/month/day minus (N-1) days
instead, which yields the first instant of that calendar day, including
zones where midnight itself is skipped by DST. The 1d rolling window,
today, all-time and custom ranges are unchanged.
Refs #8719
