Render Mermaid fenced code blocks in the viewer and HTML exports

[davidmoshal]

Problem

OpenKnowledge 0.8.4 recognizes fenced mermaid blocks, but openknowledge view displays the Mermaid source as an ordinary code block instead of rendering a diagram.

Reproduction

1. Add a Mermaid fence to a knowledge-base document:

flowchart LR
  A --> B

Loading

2. Run openknowledge view <knowledge-base>.
3. Open the document.

Observed behavior

The viewer emits:

<pre class="code-block language-mermaid" data-language="mermaid">

and displays the source text. No Mermaid runtime is loaded.

Expected behavior

Mermaid fences render as diagrams in:

• the initial viewer panel;
• dynamically opened stacked panels;
• the default interactive HTML export.

Invalid Mermaid should retain a readable source fallback and expose a useful error without breaking the rest of the document.

Rendering should work offline from same-origin assets and use Mermaid’s strict security mode.