feat: Add pace tick to the 5H/7D menu-bar usage bars

The session/weekly usage bars now show a thin vertical tick at the
fraction of the rate-limit window already elapsed (UsageWindow
.elapsedPercent + RateLimitWindow constants). Fill past the tick = you
are burning faster than the clock; behind = headroom. Original
UtilizationBar fill is unchanged (verbatim from b012f0d); the tick is
the only addition. Removed the now-dead isBar property.