Add Mermaid diagram rendering support

[tomquist]

Summary

Adds support for rendering Mermaid diagrams in the preview pane, matching how GitHub renders ```mermaid fenced code blocks.

When a user writes a fenced code block with the mermaid language identifier, the preview now renders it as an interactive SVG diagram instead of displaying the raw source.

What's included

• Mermaid library integration: Added mermaid as a dependency, rendering diagrams client-side via mermaid.render()
• Custom marked extension: A renderer extension intercepts ```mermaid code blocks during Markdown parsing and replaces them with placeholder <div> elements, which are then rendered to SVGs in a post-parse async pass
• Dark mode support: Mermaid diagrams automatically switch between default and dark themes when the user toggles dark mode, with diagrams re-rendering on theme change
• GitHub-style styling: Mermaid containers are centered with a subtle background and rounded corners, matching GitHub's rendering style
• Error handling: Invalid mermaid syntax gracefully falls back to showing the raw source with an error indicator
• Default template: Added a simple flowchart example to the default template so new users can see mermaid support immediately
• Vite code-splitting: Mermaid's diagram types are automatically code-split by Vite, so only the diagram types actually used are loaded

Supported diagram types

All mermaid diagram types are supported, including flowcharts, sequence diagrams, class diagrams, state diagrams, ER diagrams, Gantt charts, pie charts, and more.

Test plan

• Open the app and verify the default template renders the mermaid flowchart as an SVG diagram
• Toggle dark mode and verify the mermaid diagram re-renders with the dark theme
• Type a new mermaid code block (e.g., sequenceDiagram) and verify it renders live
• Type invalid mermaid syntax and verify it shows the raw source with error styling
• Verify non-mermaid code blocks still render normally as <pre><code> blocks
• Verify PDF export still works with mermaid diagrams in the preview
• Run npm run build and verify no build errors