Add a separate session context usage indicator

The current usage indicator shows monthly subscription usage, which is useful, but I would also like visibility into the context usage of the current session. I found that I can see it on the top menu, but I need to click that for me to see it.

I suggest separating these into two UI elements:

Left sidebar (account level)

Monthly subscription usage
Credits or quota consumed this month

Current session (chat level)

Context window usage for the active session
Percentage of available context currently used
Optional details such as files, conversations, or MCP resources currently contributing to context

No files or tests are named. Start by inspecting the existing top-menu usage indicator and the sidebar UI to understand how monthly subscription usage and current-session context usage are exposed. Done means the two usage concepts are visibly separate, with active-session context percentage shown and optional contributing-resource details considered.

索引モデルが issue の本文から書いたものです。