Have you searched existing Github issues to see if someone has already requested this feature?
Yes

Is your feature request related to a problem? Please describe.
Currently, when creating a code block for Mermaid syntax in heynote, the text is treated as plain text. This lack of syntax highlighting can make it difficult to write and read complex diagrams.

Describe the solution you'd like
I would like to request the addition of syntax highlighting for Mermaid code blocks. This would visually distinguish the different elements of the Mermaid syntax, making it much easier to work with.

As a further enhancement, it would be incredibly beneficial if heynote could also render the Mermaid diagrams directly within the note. This would allow for a live preview of the diagram as the code is being written, similar to how some other Markdown editors and platforms handle Mermaid integration.

Describe alternatives you've considered
The only alternative at the moment is to write Mermaid syntax as plain text within a standard code block and then use an external tool to render and validate the diagram.

Additional context
Adding Mermaid support would be a valuable feature for many users, especially those in technical fields who use diagrams to brainstorm, document processes, or visualize data. Mermaid is a popular and lightweight way to create diagrams from text and its integration would greatly enhance heynote's utility as a versatile scratchpad for developers and other power users.