📊 feat: Add zoom hint for Mermaid

[joelhirzel]

Pull Request Template

Summary

Most users likely won't notice that the inline Mermaid diagrams are zoomable using the mouse wheel. This PR adds a zoom hint to improve discoverability of the Ctrl/Cmd + Scroll zoom feature.

When users scroll on a Mermaid diagram without holding the modifier key, a hint briefly appears showing "⌘ + Scroll to zoom" (or "Ctrl + Scroll to zoom" on Windows/Linux).

Change Type

• New feature (non-breaking change which adds functionality)

Testing

Tested manually:

1. Hover over a Mermaid diagram and scroll without Ctrl/Cmd → hint appears
2. Scroll with Ctrl/Cmd held → diagram zooms, no hint

zoom-hint.mov

Checklist

• My code adheres to this project's style guidelines
• I have performed a self-review of my own code
• I have commented in any complex areas of my code
• My changes do not introduce new warnings
• Local unit tests pass with my changes