feat: Playground improvements

[RoboNET]

Summary

• Content options autocomplete — context-aware YAML autocomplete for options: block inside type: content elements based on format: value (NDC options + charset styles)
• QR code support — enabled WithQr() in Playground builder
• Barcode support — enabled WithBarcode() in Playground builder
• Render error surfacing — missing images/resources now show error in Errors tab instead of silent empty render
• GitHub Pages fixes — overwrite: true for artifacts, base href rewrite, .nojekyll, fingerprinted main.js via importmap

Test plan

• Open Playground, create type: content with format: ndc, verify options: autocomplete suggests input_encoding, columns, etc.
• Inside charsets: > X: verify autocomplete suggests font_style, color, etc.
• Render a type: qr element with data: "test"
• Render a type: barcode element
• Reference a non-existent image — verify error appears in Errors tab
• Deploy to GitHub Pages — verify site loads correctly