Chat: add a per-chat auto-scroll mode with limited streaming preview

## Problem

When an agent streams responses or progress updates, the active chat continuously scrolls to the newest content. This makes it difficult to keep reading an earlier part of a long conversation.

## Proposal

Add a per-chat auto-scroll control, ideally as an icon in the chat toolbar. It should be possible to switch modes at any time:

- **Follow latest**: Current behaviour; the chat follows streamed content automatically.
- **Reading mode**: When a new response starts, reveal only a configurable number of its first lines. Do not scroll further while the response continues streaming. The user decides when to continue by scrolling down manually.

When the user sends a new prompt, the chat should automatically move to the first lines of the next response while still respecting the selected reading mode.

## Suggested details

- Keep the mode local to the current chat session.
- Make the current state visually clear and accessible, including a tooltip and keyboard access.
- Allow the preview size to be configurable, for example 2, 5, or 10 lines.
- Keep a **Jump to latest** action available so users can immediately resume following the stream.

## Motion and streaming behaviour

When **Follow latest** is enabled, scrolling should be smooth and coalesced rather than jumping instantly for every streamed line or token.

- Batch streaming updates before adjusting the scroll position.
- Animate the resulting movement smoothly instead of issuing a scroll jump for each output update.
- Respect the user's reduced-motion preference and provide an option to disable animation.
- In **Reading mode**, do not move the viewport after the configured preview lines have been revealed.

## Why this matters

This gives users situational control: live monitoring while work is in progress, or a stable reading experience for long responses. It is especially helpful for users with low vision, cognitive load concerns, or limited screen space.