chore: add Biome formatter and fix index.html CDN import

[thomas-hervey]

Two small housekeeping items to address together:

1. Add Biome — the project has no formatter or linter. Add Biome (single tool for both formatting and linting, Rust-based) with a format npm script and consistent config.

2. Fix index.html CDN — the demo page imports from `https://cdn.skypack.dev/arc@1\` which 404s (v1 not published). Update to a working CDN reference or switch to a local build import.

These are purely cosmetic/developer-experience changes with no impact on library logic.

[thomas-hervey]

This is addressed in my fork's biome-formatter branch.
I'm waiting for #77 and #76 to land before rebasing, formatting, and opening up a PR.

[jgravois]

personally I think a 1.x skypack link is better quickstart DX than a local link, but I don't feel strongly about it.

no reason to put off publishing either way imo.