Add scroll-to-latest button while reading older chat messages

Bump eca-webview to pick up a new floating "scroll to latest" button
that appears when the user has scrolled away from the bottom of the
chat (e.g. after a page reload while messages are still streaming in).
Clicking it smooth-scrolls to the bottom and re-arms the existing
auto-stick-to-bottom behavior so subsequent streaming chunks keep the
view pinned, until the user scrolls again. No host-side changes are
needed — the bridge layer is unaffected.

Related to editor-code-assistant/eca-webview#9.

🤖 Generated with [eca](https://eca.dev)

Co-Authored-By: eca-agent <git@eca.dev>

Author: ericdallo