feat: Add mermaid diagram support

[SethCalkins]

Summary

Adds support for rendering Mermaid diagrams in markdown files.

Changes

• Add mermaid.min.js library (v11.x) to Resources
• Detect mermaid code blocks (language-mermaid or class="mermaid")
• Embed mermaid.js inline in HTML for compatibility with macOS 12+ data-based Quick Look preview
• Enable JavaScript in WKWebView when inlineImageExtension OR unsafeHTMLOption is enabled (changed from AND to OR)

Why embed inline?

macOS 12+ uses data-based Quick Look preview (QLPreviewReply) which cannot access file:// URLs due to sandboxing. By embedding mermaid.js inline in the HTML, the diagrams render correctly regardless of the preview method used.

Supported syntax

Fenced code blocks:

```mermaid
graph TD
    A[Start] --> B{Decision}
    B -->|Yes| C[OK]
    B -->|No| D[Cancel]
```

HTML divs:

<div class="mermaid">
graph TD
    A --> B
</div>

Features

• Automatic dark/light theme based on system preference
• Supports all mermaid diagram types (flowcharts, sequence diagrams, class diagrams, etc.)
• Only loads mermaid.js when mermaid blocks are detected in the markdown

Test Plan

• Verify mermaid diagrams render in Quick Look preview
• Test with both light and dark system themes
• Test flowcharts, sequence diagrams
• Test on different macOS versions (11, 12+)

[SethCalkins]

@sbarex please review for merge