Good news (everybody)! I’ve officially removed the email requirement and implemented a decentralized sync engine that keeps to my 'no-server' north star.
I ended up using Nostr as the backbone. Here is how it works now:
No Accounts: Your identity is a cryptographic key pair generated locally. No emails, no passwords.
Privacy-First Sync: When you want to share with a partner, you create a 'Vault.' This generates a shared secret key (NIP-44) that you share via a QR code.
Decentralized: Data is encrypted on your device and synced via decentralized relays. Even the relay operators can't see your baby's logs—it's just gibberish to them.
Self-Hosting Friendly: Since it uses Nostr, you can even point the app to your own private relay if you want 100% control over the infrastructure.