feat: add mermaid diagram support

[MassimilianoPili]

Adds rendering of mermaid diagrams from mermaid fenced code blocks, toggleable
via the new "mermaid" markdown extension.

How it works

• A markdown-it fence override emits

   for mermaid blocks.

• The runtime loads on demand and runs after each preview render via the existing
  MarkdownRenderContext.postRender hook. Documents without a diagram never load it.
• Runtime handling mirrors highlightjs: mermaid is an npm dependency and its prebuilt
  UMD is copied into dist/js at build time (build:mermaid), then deployed to
  assets/markdown by the existing packaging step. No runtime blob is committed.
• Diagram theme follows the preview: dark is detected from the injected markdown-dark stylesheet.

Why a copy and not a webpack entry: mermaid lazy-loads diagram types with dynamic
import(); bundling would emit chunks the runtime loader never fetches. The published
UMD already inlines everything and assigns window.mermaid.

Enable via Settings, checking "mermaid" in the Markdown Plugins list.