End-to-end encrypted secret sharing with the Web Crypto API

I got tired of using onetimesecret.com so I built my own secret sharing service. It has zero dependencies, only standard Web APIs, and the backend never stores any plaintext because everything is end-to-end encrypted (E2EE). The backend only sees the ciphertext and some metadata, and keeps them for a limited amount of time in a Redis (or Valkey) configured to never persist anything on disk.