Tidy: stale docs, unused dependencies, duplicate formatting config #86

Found in the 12 October audit.

• Stale docs: README, docs/toolchain.md and CONTRIBUTING describe steps and tools that have changed, such as the SvelteKit-era commands and the release flow. Bring them in line with the Astro site and the staging → main flow.
• Unused dependencies: react and react-dom in package.json. Remove them if nothing imports them. Check the build output and the @bundu/ui peer dependencies first.
• Two formatters: prettier and Vite+'s vp fmt both format. Pick one, the org's, and keep its config. format:check and vp fmt must agree.