fix(chat): name the users a join notice is about

[bmc08gt]

System messages showed only the server's fallback_text, which names no one. SystemContent.user_joined carries the user who joined and, for an admitted join, who let them in, but ProtobufToLocal dropped it.

• MessageContent.System now has an optional SystemEvent.UserJoined(userId, admittedBy). DIRECT and an unset method map to a plain join; a missing user_id or an unknown type maps to no event. The reverse mapper writes it back.
• The stored JSON gains two optional hex fields, so no Room migration. Rows stored before this have neither and keep showing the fallback text.
• displayText words the notice from current profiles: "Alex joined the group", "Sam added Alex", and "you" for the viewer ("You joined the group", "Sam added you", "You added Alex"). If a name it needs isn't known yet, the row reads as the fallback text.
• The transcript names users from the group's sender profiles and requests any it is missing, the same way quoted authors are named. The chat list preview uses the same wording, and unnamedGroupSender asks for the users a notice names, since a notice has no sender.

iOS has the same gap (ConversationMessage.swift maps every system type to its fallback text); that change will go in its own PR.