Feature: Usage graph for COST, and other small improvements to Usage screen

The USAGE screen cycles between the Metrics TOKENS, REQUESTS, ERRORS, and LATENCY. The display below shows that it also knows about COST. That should be one of the metrics in the cycle.

Also

• The Units don't display on the vertical access or anywhere else, even though there is room.
• The Session ID appears twice, even though it doesn't need to
• The Sort order, e.g. [sort: TIME▼] appears, even though it has no meaning on this screen
• There is an indication of 1.0 ev/s drops: 0, but I don't know what they mean
  • Claude says "drops is dead — it can only ever display 0"
  • weather-agent/demo-with-abctl.md: says "The bottom-footer rate indicator (3.2 ev/s) is a live smoothed events-per-second gauge — useful to tell if traffic is flowing or something upstream is stuck."

Start by locating the USAGE screen renderer and the code that cycles through TOKENS, REQUESTS, ERRORS, and LATENCY; also read weather-agent/demo-with-abctl.md for the event-rate indicator context. Done means COST appears in the cycle and the requested units, Session ID, sort order, and rate/drop indicators are reviewed and adjusted as specified.