ui: no time zone setting for the install itself #43

Every time the web interface shows — a chart's day buckets, a preset like "Today" or
"Yesterday", a picker's default range — uses the browser's own time zone. There is no
install-wide time zone setting; the API's windows are UTC, and a preset is computed from
whichever browser is asking.

That is deliberate for a single admin on their own laptop, but it has a real cost for
anything else: two people looking at the same install from two zones see different day
boundaries for the same report, and a shared screen (a dashboard on an office TV, a kiosk,
a scheduled screenshot job) reports whatever zone the machine running the browser happens to
be set to, which is not necessarily a zone anyone chose for the reports themselves.

A day chart's buckets already take an offset parameter — a whole number of hours from UTC
midnight — because the hourly rollups force day boundaries onto whole-hour lines. The
interface currently computes that offset from the browser's own zone. Adding an install-wide
time zone (stored the same way the traffic settings and retention settings are) would let an
operator pin it once, and the interface would send that offset instead of deriving one from
whichever machine is looking at the screen. A per-browser override could stay as a
preference layered on top for the single-admin case this already serves.