fix: render Mermaid diagrams in PDF export

[aniongithub]

Problem

PDF exports were showing Mermaid diagrams as raw code blocks instead of rendered SVG flowcharts. The headless Chromium (Playwright-bundled) in the devcontainer cannot fetch external CDN resources, so the previous approach of loading Mermaid via a CDN <script type="module"> silently failed.

Solution

Serve mermaid.min.js from the same local HTTP server that serves the HTML document to chromedp. The bundle is located at runtime from webui/node_modules/mermaid/dist/mermaid.min.js. When not found, the export degrades gracefully (shows code blocks as plain text).

Changes

internal/share/pdf.go

• Replace CDN ES module import with local /mermaid.min.js script tag
• Add findMermaidJS() to locate the Mermaid bundle from common paths (project root, relative to package dir, relative to executable)
• Add pagesHaveMermaid() to detect mermaid blocks and skip loading when unnecessary
• Serve mermaid.min.js from the local HTTP server alongside the HTML
• Add data-mermaid-done signal: set immediately when no mermaid, or after rendering completes

internal/share/pdf_test.go

• Rewrite TestPDFMermaidRendering to verify actual SVG rendering via chromedp (checks for <svg> elements, flowchart class, and removal of original code block)
• Add TestPDFRenderHTMLNoMermaid to verify no mermaid script is included for plain pages
• Update TestPDFRenderHTMLContainsMermaidScript for new local-server approach

How it works

1. pagesHaveMermaid() scans page bodies for ```mermaid blocks
2. If found, findMermaidJS() reads the ~3MB UMD bundle from disk
3. htmlToPDF() starts a local HTTP server with two routes: / (the HTML) and /mermaid.min.js (the bundle)
4. The HTML includes a <script> that initializes Mermaid, replaces <code class="language-mermaid"> blocks with <div class="mermaid">, calls mermaid.run(), then signals completion
5. chromedp polls for data-mermaid-done="true" before printing to PDF

Testing

All tests pass including the new mermaid rendering verification:

ok  github.com/aniongithub/mind-map/internal/share  0.677s