Fix race condition in mermaid observer

[william-allspice]

This Pull Request addresses a race condition in the updateIframeHeight function where it is sometimes called when the iframe is not fully loaded or accessible resulting in an alarming error message for the user.

I have reproduced this here: https://gitea.com/small_world/MermaidTest/src/branch/main/

Please note: This error is reproducible but it happens intermittently.

To address this we:

1. Add defensive programming within the updateIframeHeight function
2. Delay instantiating the intersection observer until the iframe has loaded