Show agent context usage as a circular progress indicator

[uzayer]

In the agent panel’s context/tokens-and-price status area, replace the speed icon with a compact circular progress indicator that visually represents context-window usage, similar to Cursor.

Acceptance criteria:

• The indicator uses the existing used-context and total-window values.
• Its fill/progress proportion reflects usage accurately and remains legible at small sizes.
• Provide an accessible text label/tooltip with used tokens, total context, and percentage.
• It updates during an active agent run without layout shift.
• Preserve the existing token and price information; gracefully handle agents that do not report context limits.