Calm the usage popover and settings card: smaller, muted provider logo
and name, more space between the header and the usage bars.
The popover divider used the `border` token, which equals the popover
background in dark themes and was invisible. Switch it to `borderAccent`
(the token the popover outlines itself with) and span it edge to edge.
Shrink the context meter ring and, while a session is active but usage
hasn't arrived yet, render a track-only ring in the final footprint so
the real ring fades in without shifting siblings. Agents that never
report usage still render nothing.
Mohamed Boudra
3493e0492d