Add a "keep screen awake" toggle to prevent the display/computer from sleeping

### Feature summary

A "keep awake" toggle/button that prevents the screen and machine from sleeping while it's on.

### What problem are you trying to solve?

When I want the screen to stay on (e.g. while watching a long agent run or keeping a session visible), there is no built-in way to stop the display/computer from sleeping. My current workaround is to open a 1-hour black YouTube video fullscreen just to keep the screen awake, which is clunky and wasteful.

### Proposed solution

Add a simple toggle in the app (e.g. in the header or a tray/menu item) that keeps the screen and system awake while enabled, and releases the lock when turned off or the app closes. On macOS this is the equivalent of holding a `caffeinate` assertion (prevent display + idle sleep).

### Workflow impact

Removes the need for hacky workarounds (black YouTube video), and lets me keep a session or long-running agent visible without the screen turning off.

### Installation context

GitHub Copilot desktop app on macOS (personal machine).

### Additional context

A small always-visible state would help (so I can tell at a glance whether keep-awake is currently on).

Start by checking the desktop app's macOS integration and the proposed header or tray/menu entry points; the issue provides no file or test paths. Compare the keep-awake behavior with a macOS caffeinate assertion, then verify that enabling, disabling, and app closure release it and that the state is visible.

Written by the indexing model from the issue text.