feat: add echarts-in-chat desktop plugin

[Tommy00748]

What

Add echarts-in-chat — a desktop plugin that renders ```echarts code blocks in the Hermes Desktop conversation stream as live, interactive charts (zoomable, legend-filterable, resize-aware), replacing the static code block in place.

What it demonstrates

• Message-stream DOM injection — turning a rendered code block into a live component inside the chat (a pure-frontend plugin surface not yet covered in this repo).
• Reload-safe singleton — survives repeated ⌘K reloads without stacking duplicate observers or double-processing blocks.
• Race-free claiming — pre.replaceWith(div) runs synchronously in the scan callback, so stale plugin instances can never steal a block mid-await.
• Shiki integration quirks — content sniffing (shiki renders no language label), the 120px scroll-wrapper trap, and full-body scan + debounce + interval backstop for async highlighting.
• Blob-URL reality — disk plugins execute from a blob:file:// URL where relative imports fail; ECharts loads via CDN with an optional local vendor path.

Usage

Write a code block with language echarts whose content is a valid ECharts option JSON:

```echarts
{ "xAxis": { "type": "category", "data": ["Mon","Tue"] },
  "yAxis": { "type": "value" },
  "series": [{ "type": "bar", "data": [120, 200] }] }
```

The block is replaced by a 400px-tall interactive chart ("hermesHeight" in the option overrides, clamped 280–560px).

Notes

• Pure ESM, no build step; imports only what the disk loader allows (@hermes/plugin-sdk / react / react/jsx-runtime) — in practice it does pure DOM work and needs none of them.
• ECharts 5.5.0 vendored (Apache-2.0) with jsDelivr CDN fallback.
• Tested on the macOS desktop app (v0.20.x) through multiple plugin reload cycles.