Make Steer and Queue visible choices before sending

### Feature summary

Show a visible choice between steering the current work and queueing the next message.

### What problem are you trying to solve?

When I wanted to send another message while Copilot was working, I could not tell whether it would change the current task or wait until it finished. I ended up not sending it because I did not want to disturb the work in progress.

I only later learned that Enter steers and Ctrl+Enter queues. Queueing already exists, but this choice is difficult to discover unless you already know the shortcut.

### Proposed solution

While Copilot is working, show both actions in the Send button or its dropdown:

- Steer current work
- Add to queue

The existing shortcuts can remain as they are.

VS Code already exposes the same choices while a request is running:
https://code.visualstudio.com/docs/chat/chat-overview#_send-messages-while-a-request-is-running

### Workflow impact

I could capture next steps immediately without accidentally redirecting the current task or having to remember to return later.

### Installation context

GitHub Copilot app 1.0.23 on Windows x64, OS build 26200.8655, in the /chat view.

### Additional context

Related: #27 introduced queueing, #275 explains the current shortcut, and #1757 shows that queueing can still be difficult to discover.

This request is specifically about making the existing choice visible before sending.

Commencez par inspecter la vue /chat et le comportement actuel de mise en file d’attente, puis comparez l’interaction demandée avec la référence VS Code liée. Le travail est considéré comme terminé lorsque « Steer current work » et « Add to queue » sont visibles pendant que Copilot travaille, tout en conservant le fonctionnement des raccourcis clavier existants.

Rédigé par le modèle d'indexation à partir du texte de l'issue.