[Bug]: Plan hides conversation

### Platform

macOS

### Operating system version

macOS Tahoe 26.3.1 (a)

### System architecture

ARM64 (M1, M2, etc)

### PolyScope Version

0.19.0

### Bug description

When a plan is being displayed, you cannot scroll up to view the conversation.

This is particularly problematic when you propose changes to a plan. You might want to see the agent's thinking or responses. At the moment you just get the new plan plopped in front of you again.

Also, there's no way to just ask the agent something. You are forced to update the proposal.

### Steps to reproduce

- Make a plan
- Try to view the previous conversation or ask it a question without "proposing changes".

### Relevant log output

```shell

```

未指定文件、测试或入口点。首先在 macOS Tahoe 上使用 PolyScope 0.19.0 显示计划，以复现该问题，然后检查计划和对话 UI。完成的标准是：查看计划时仍可访问之前的对话，并且可以向 agent 提问，而无需强制更新提案。

由索引模型根据 Issue 内容生成。