Desktop app: persistent context/token usage indicator like the CLI status line

## Summary
The CLI shows a persistent status bar at the bottom of the terminal with model name, context window usage (%), and elapsed time. The desktop app's Code tab has no equivalent — context usage is only visible by clicking the usage ring next to the model picker, or via `/usage`.

## Request
Add an optional persistent indicator in the desktop app (e.g. a small always-visible context-usage % near the model picker, or a configurable status bar) so users can monitor remaining context at a glance without clicking through, similar to the CLI's `statusLine` behavior.

## Context
Noticed while comparing the CLI terminal UI (which shows e.g. "7d 13%" for context remaining) against the desktop app, which requires an extra click to see the same info.