fix: render mermaid diagrams instead of raw code blocks

[whg517]

Summary

docs/architecture.md and its Chinese translation contain 8 ```mermaid fenced blocks, but @docusaurus/theme-mermaid was never installed and `markdown.mermaid` was never enabled. Docusaurus fell back to treating them as plain code, so the architecture diagrams added in #31/#32 currently ship to the site as raw mermaid source text.

Changes

• Install @docusaurus/theme-mermaid@3.10.1 (pinned, matching the other @docusaurus/* deps)
• Enable markdown.mermaid and register the theme in docusaurus.config.ts
• Map the diagram theme to the site colour mode ({light: 'neutral', dark: 'dark'}) so diagrams do not stay light on a dark page

Testing

• npm run build passes (both locales)
• npx tsc --noEmit passes
• No lines exceeding 200 characters
• Verified in a browser: all 4 diagrams on /docs/architecture render as SVG
  • dark mode: node fill rgb(31,32,32), label rgb(227,227,227)
  • light mode: node fill rgb(238,238,238), label rgb(51,51,51)
• Neither locale's build output contains a language-mermaid block any more (was 8, now 0)

Note

package-lock.json grows a lot because mermaid pulls in d3/cytoscape/dagre. 27 of the deleted lines are incidental "peer": true markers from an npm version difference, not a dependency change.

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