Test hook: a synthetic peer for single-device UI testing

Checklist

• I have used the search function for OPEN issues to see if someone else has already submitted the same feature request.
• I have also used the search function for CLOSED issues to see if the feature was already implemented and is just waiting to be released, or if the feature was rejected.
• I will describe the problem with as much detail as possible.
• This request contains only one single feature, not a list of multiple (related) features.

App version

1.7.4

Feature or improvement you want

A debug test hook that creates a synthetic peer the chat UI can interact with: a name, a message from them in the chat, and a private conversation that can be opened. Debug builds only, next to the existing hooks.

The existing inject_peers command adds bare peer IDs, which is enough for the peer counter and notifications. The chat UI needs more: mention suggestions come from names, tapping a sender needs a message from them, and the private chat sheet needs a conversation.

Why should this be added?

Peer-dependent UI can only be tested with two devices. #847's send-path fix could be walked on an emulator, but its mention half could not: with no peers there are no mention suggestions. Neither #854 fix could be walked: there is no sender name to tap and no conversation to switch into.

Screenshots / Drawings / Technical details

It could be built at three sizes: just a name, a name plus a message from the peer, or those plus a chat thread that can be opened. This does not ask to fake the encryption; the existing commands cover that with real devices.

If one of these is agreed, I can build it.