feat(kimi-web): add a collapse button at the bottom (or floating) of long expanded blocks

Problem

In the kimi web session view, several blocks are collapsible (thinking, work-in-progress groups, tool call details, etc.). When such a block is expanded and its content is very long (long thinking, large tool output), the collapse control only exists at the top of the block. To collapse it again, the user has to scroll all the way back up, which is tedious.

Expected behavior

Either of the following would work:

1. Render an additional "collapse" button at the bottom of an expanded block; or
2. When the block header scrolls out of the viewport, show a floating "collapse" button (sticky/floating) that collapses the block and scrolls the view back to the block header.

Environment

• kimi web 0.37.2
• macOS, Chrome

Additional context

It would be even better if the long-content collapse interaction were considered consistently across TUI and web. Thanks!

Start in the Kimi web session view by locating the existing collapsible block headers for thinking, work-in-progress groups, and tool call details. Inspect how expanded long content is rendered, then add a bottom or floating collapse control and verify that long thinking and tool output can be collapsed without scrolling back to the header.