Settings gains a Usage Refresh picker mirroring the quota cadence
pattern. Auto keeps the adaptive default (30s active, battery and Low
Power backoff); fixed cadences ignore power state but an open popover
always gets the active 30s; Manual never auto-spawns, refreshing only
on popover open, Refresh Now, and first launch (wake included: manual
wakes run stuck-state cleanup without spawning). Stored in UserDefaults
as CodeBurnMenubarRefreshSeconds (-1 auto, 0 manual, else seconds) and
documented in the README with the defaults-write escape hatch.
Refs #647
