feat(mermaid): expose header slot controls

[cc-hearts]

🤔 This is a ...

• 🆕 New feature
• 🐞 Bug fix
• 📝 Site / documentation improvement
• 📽️ Demo improvement
• 💄 Component style improvement
• 🤖 TypeScript definition improvement
• 📦 Bundle size optimization
• ⚡️ Performance optimization
• ⭐️ Feature enhancement
• 🌐 Internationalization
• 🛠 Refactoring
• 🎨 Code style optimization
• ✅ Test Case
• 🔀 Branch merge
• ⏩ Workflow
• ⌨️ Accessibility improvement
• ❓ Other (about what?)

🔗 Related Issues

N/A

💡 Background and Solution

Custom Mermaid headers fully replace the default controls, leaving custom header implementations unable to reuse render-type switching, zooming, reset, download, or source-copy behavior.

Expose a typed MermaidHeaderSlotScope to the header slot with renderType, setRenderType, zoomIn, zoomOut, resetZoom, download, and copy. The existing default header remains unchanged, and image-only controls are no-ops in code mode.

The custom header demo now uses the slot scope, and the API documentation describes the full contract in English and Chinese.

📝 Change Log

Language	Changelog
🇺🇸 English	Mermaid: expose built-in controls through the header slot scope.
🇨🇳 Chinese	Mermaid：通过 header 插槽作用域暴露内置操作能力。