[feature]: Support Mermaid diagrams inside Markdown (GitHub-style)

Is there an existing issue for this? • I have searched the existing issues Summary It would be extremely helpful if Plane supported Mermaid diagrams directly inside Markdown content, similar to how GitHub and other tools do. Example syntax: Why should this be worked on? Today, if we paste Mermaid code blocks in Plane, they are rendered as plain text instead of diagrams. Native rendering would make specs, architecture documents, and flows much easier to understand and maintain. Why this matters • Improves documentation quality without leaving Plane • Makes it easier to share system architecture, flows, and dependencies • Reduces the need for external tools (Miro, draw.io, etc.) just to keep diagrams updated • Fits naturally into Markdown-heavy workflows that many dev teams already use Suggested behavior • Detect `mermaid code blocks in Markdown • Render them as SVG/canvas diagrams • Fallback gracefully to plain…