Comment by lukeschlather

    const encrypted = await window.crypto.subtle.encrypt(
      { name: "AES-GCM", iv: new Uint8Array(12) /* don't reuse key! */ },
      key,
      new TextEncoder().encode(JSON.stringify(content))
    );

This looks wrong. The iv should be a randomly generated string that is only used once. I'm not super familiar with modern Javascript, but I think you're just initializing a bunch of null bytes.

Honestly, I feel like whoever designed AES intended for people to make this mistake, because half of the sample code I see has this error, and it would have been easily avoided by specifying the IV length in the standard and requiring libraries to automatically generate it and prepend it to the ciphertext rather than letting callers have control over how it is initialized and stored.