fix: render extension widgets next to the editor

[kaiwishc]

Summary

aboveEditor and belowEditor refer to the chat editor, but pi-web rendered those widgets in the scrollable message list. This moves them into the editor area, so they stay visible while the user reads a long conversation.

• aboveEditor widgets render directly above the chat input.
• belowEditor widgets render directly below the chat input.
• Empty sessions use the same layout.

Testing

• Confirmed that editor widgets are outside the scrollable message container.
• node_modules/.bin/tsc --noEmit
• npm run lint

Scope

This changes widget placement only. It does not change the extension widget API or its server-side rendering and lifecycle behavior.