[Requests and ideas] Show audio playback indicator on tabs with active preview audio

**Mood:** 😔
**Category:** Requests and ideas

When working with many open sessions in GitHub Copilot App, it can be difficult to identify which tab is currently playing audio from a previewed web application. See video below.

https://github.com/user-attachments/assets/19082cd3-1343-4c29-a24e-d215e26a0328

After navigating away from a preview, audio may continue playing in the background, but there is no visual indication of which tab is responsible for the playback. We could add an audio playback indicator to tabs containing active previews that are producing sound, similar to the speaker icon shown in Chrome and other browsers

---
| Field | Value |
| --- | --- |
| App version | 1.0.1 |
| OS | macOS 26.5.0 |
| Theme | GitHub |
| Path | /chat |
| Tenure | Week 2 |

该 issue 指出 GitHub Copilot App 中的预览标签页是受影响的区域，但没有指定实现文件或测试。首先定位预览标签页 UI 和音频播放处理，然后定义在导航后音频继续播放时指示器的行为。完成标准是：正在主动播放预览音频的标签页显示清晰的播放指示器。

由索引模型根据 Issue 内容生成。