[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

```

No files, tests, or entry points are named. First reproduce the issue on macOS Tahoe with PolyScope 0.19.0 by displaying a plan, then inspect the plan and conversation UI. Done means previous conversation remains accessible while viewing a plan and the agent can be asked a question without forcing a proposal update.

索引モデルが issue の本文から書いたものです。