fix(mermaid): text invisible due to DOMPurify stripping style & foreignObject (#119)

DOMPurify.sanitize() removes <style> blocks and strips all content
inside <foreignObject> elements. Mermaid v11+ relies on both for
correct text and node rendering, causing invisible text in the editor.

Remove DOMPurify sanitization for Mermaid SVG since the output is
generated by the trusted mermaid library from validated diagram syntax.