Prototype 0b — loud-failure upload

Pick any file. The browser hashes it, the server stores it in the dashboard's private Blob store, reads it back, and compares hashes. Green only when the read-back matches. To test the failure mode: start an upload of a big file, kill the VPN mid-flight, and this page must go red — never silent. Keep the file under 100MB, or Vercel rejects it before this app is involved and you test nothing.