Stuck on Decrypting mailbox

**Describe the bug**
Initial setup of Criptext on macOS as a second device does not sync mails.

**To Reproduce**
Steps to reproduce the behavior:
1. Sign up to criptext on iOS and finish setting it up.
2. Download criptext for macOS.
3. Sign into criptext for macOS and authorize this device from the iOS device (I had 2 factor authentification enabled)
4. criptext for macOS will generate keys, exchange them with iOS criptext, get mail archive and start decrypting them.
5. I tried multiple times and the last time left it running for 6 hours overnight, with no progress: It's still stuck at 90%.

**Expected behavior**
Syncing a new nearly empty inbox should be fast and painless.

**Screenshots (Optional)**

**Device and System information (please complete the following information):**
- Device: MacBook7,1 (2010 white unibody)
- OS Version: High Sierra 10.13.16
- App Version: 0.29.0

Inizia riproducendo la configurazione del secondo dispositivo macOS descritta nell’issue e osserva in quale punto la decrittografia della mailbox rimane al 90%. La payload non indica file sorgente, test o entry point, quindi traccia il flusso di configurazione, scambio delle chiavi, download dell’archivio e decrittografia. Il lavoro è completato quando un dispositivo macOS appena autorizzato completa la sincronizzazione di una inbox di piccole dimensioni.

Scritto dal modello di indicizzazione a partire dal testo della issue.