Make the Zashi seed phrase entry UI similar to that used by the Keystone hardware wallet

[nuttycom]

Is your feature request related to a problem? Please describe.

The UX of Keystone seed entry is really lovely: it provides a custom keyboard that only enables the keys that correspond to a valid next letter in one of the seed phrase wordlist words, and a suggestion pane that is populated only with those words. It displays 24 individual word slots; it's really slick and helps to avoid errors in seed phrase entry.

Describe the solution you'd like

Basically provide the same sort of UI in Zashi. It's great, let's just copy it.