Fork choice viz: add toggle to stop auto-tracking the head #149
Description
From PR #142 review comment:
The "scroll to head" feature is bad when you're trying to see old blocks. We should add a way to stop tracking the head.
File: crates/net/rpc/static/fork_choice.html
When viewing the fork choice tree visualization, the auto-scroll-to-head behavior makes it difficult to inspect older blocks.
Proposed behavior: Automatically disable head tracking when the user scrolls up (away from the head). This way users can freely explore old blocks without fighting the auto-scroll, and re-enabling tracking can be done by scrolling back to the head or clicking a "follow head" button.