Scroll control during streaming responses: no way to jump to or pause at the start of the response

## Problem

During a streaming AI response, the view **auto-scrolls through both the thinking text and the actual response** as content is generated. This makes it difficult to read the response from the beginning, since the view keeps moving to follow new content.

There is no easy way to get back to the start of the response once the user wants to read it from the top.

## Expected behavior

The user would like one of the following:

1. **A button to jump back to the beginning of the response** — so the user can easily get to the start and read through it.

2. **Stop and wait at the beginning of the response, and let the user scroll down when ready** — after the thinking text finishes, pause at the start of the actual response and let the user control scrolling from there instead of auto-scrolling.

## Context

- The issue is most noticeable on mobile, but applies generally.
- This is a common pattern in chat/agent UIs (e.g. a "jump to latest" or "pause autoscroll" affordance).

The issue names no files or tests; start by locating the streaming response view and its auto-scroll behavior. Compare the two expected interaction options, especially on mobile, and verify that users can read from the response start without unwanted scrolling.

Written by the indexing model from the issue text.