feat(plugin-mermaid): add mermaid diagram rendering plugin

[tigeryao123]

Summary

Add @floatboat/nexus-plugin-mermaid — renders fenced mermaid code blocks as inline SVG diagrams.

Changes

• New package: packages/plugin-mermaid/
• Async SVG rendering with loading placeholder and error handling
• Hover-to-reveal ✎ edit button (matches plugin-math UX)
• 18 unit tests, all 555 project tests pass
• Integrated into electron-demo

Test Plan

1. Open electron-demo, type a mermaid code block, move cursor away → diagram renders
2. Click diagram → returns to edit mode to reveal source
3. Type invalid mermaid syntax → shows inline error message

Checklist

• New package follows existing plugin conventions
• Tests pass (pnpm exec vitest run)
• Build succeeds (pnpm build)
• Manual verification in electron-demo

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