Settings page uses Windows-style close button instead of macOS convention

## Short summary

Settings page dialog uses a Windows-style ✕ close button in the top-right corner instead of following macOS UI conventions.

## Affected version or release

v0.2.6

## Installation context

macOS desktop app

## What happened?

The Settings page opens as a modal/panel with a close button (✕) positioned in the **top-right** corner. This follows Windows/web conventions, not macOS Human Interface Guidelines. On macOS, close controls belong in the **top-left** corner (traffic-light buttons).

## Steps to reproduce

1. Open the GitHub Copilot app on macOS
2. Click the settings gear icon
3. Observe the ✕ close button in the top-right corner of the Settings panel

## Expected behavior

Follow macOS HIG — use a native close button (red traffic-light) in the top-left, or the appropriate macOS sheet/panel dismissal pattern.

## Additional context

The app's own title bar correctly shows macOS traffic-light buttons — the Settings modal is inconsistent with both the app and the platform.