Always visible context window status

### Describe the feature or problem you'd like to solve

I'd like to see the current context usage permanently in the Copilot UI, like the "remaining reqs".

### Proposed solution

Getting towards the end of the context window degrades LLM performance. The Copilot CLI doesn't give any indication when that's the case, however. Similar to "_Remaining reqs.: 85.4%_" in the bottom right, there could be an indicator like "_67k/160k tokens (42%)_".

### Example prompts or workflows

_No response_

### Additional context

Inspired by this video (timestamp 11:33):