Make the "encrypt/decrypt" section usable as a minimal messenger

Current decryption demo only allows decrypting the message you encrypted to yourself. This doesn't really show off the feature.

The sections should be like this:

```
[get encryption key]
encryption key output
-------
type message
recipient encryption key
[encrypt message]
encrypted output (full, copyable)
--------
type encrypted input
[decrypt message]
decrypted output
```

Start at the existing encrypt/decrypt section in the test dapp and trace its current self-encryption and decryption flow. Rework the interaction to support a recipient encryption key and separate encrypted input, then verify that the full encrypted output is copyable and that the requested encrypt/decrypt sequence works end to end.

Written by the indexing model from the issue text.