Introducing charts to the blog

You can now embed nice-looking charts directly in posts using a fenced code block:

Bar charts work too:

And a simple pie chart:

To add one to your own post, use the 📊 Insert chart button in the writing desk, or write a fenced block with language chart containing a JSON Chart.js config:

```chart
{
  "type": "line",
  "data": { ... },
  "options": { ... }
}
```

The build script converts these into <canvas> elements that Chart.js renders on page load, and charts respect the site's light/dark theme.