

Binaryeye supports both decoding and encoding via Share menu. So you can share an image from any app to decode(can also be a photo that has a qr code somewhere). It also supports sharing a text or link to generate one. It also has searchable history support.
Other people already mentioned Roundcube and Snappymail, which are good options already.
There’s also Cypht if you want a different approach. It combines multiple accounts into same interface so you can have a unified inbox.