Support `--hidden-recipient` in the WebUI, at least for decryption

Since we only have only one PGP key, it should be simple to try it at least for decryption?

(right now we get:
```
Error: you cannot process a message that was encrypted for someone else.
```
)

Start by locating the WebUI decryption entry point and the handling for messages encrypted to a hidden recipient. Reproduce the reported “you cannot process a message that was encrypted for someone else” error with a hidden-recipient message. Done means the WebUI can decrypt such a message when the available PGP key matches, without breaking ordinary decryption.

Written by the indexing model from the issue text.