Upload Element Previews

1. Identify the Element slug. Ask if it is ambiguous.
2. Choose the source:
   • Use render for fresh files in .element-previews.
   • Use submission for committed files in static/elements.
3. Upload the previews:

cd packages/docs
bun run upload-element-preview \
  --element=<category>/<slug> \
  --source=<render|submission>

Each upload uses a new random URL postfix, including when the Element already has hosted previews. This keeps previews from concurrent branches independent.