feat: add scroll-to-bottom control for chat sessions

[oa1mgo]

Summary

Fixes #59.

This PR updates the chat scroll behavior so the view no longer jumps to the bottom after a response completes when the user has scrolled away from the latest message.

It also keeps the chat pinned to the latest message when new or streaming content arrives while the user is already at the bottom.

It adds a lightweight scroll-to-latest control that appears when the user is away from the bottom of the conversation, and removes the previous running-state bottom spacer that pushed the message list upward while the agent was active.

Changes

• Track whether the chat viewport is near the bottom before auto-scrolling
• Preserve the user's reading position while an agent response is streaming or completing
• Keep the chat pinned to the latest message when new or streaming content arrives while the user is already at the bottom
• Remove the running-state bottom spacer and last-user-message scroll anchoring
• Add a centered scroll-to-bottom button for returning to the latest message
• Hide the button naturally once the viewport reaches the bottom

Testing

• node_modules/.bin/tsc --noEmit
• npm run lint
• Manually verified the scroll button appears, scrolls smoothly to the latest message, does not flicker on click, and preserves bottom auto-follow for new messages