Disappearing / expiring messages by default in all new chats (private and public)

Signal recently added a setting to start all conversations with disappearing messages enabled by default. The default expiration time can be configured as well.

It would be great if Keybase offered the same for DMs and for public chats.

Especially after Signal rolled out this feature, I keep forgetting to set the timeout manually in apps that require manual intervention every time a conversation is started.

I use expiring messages for 100% of conversations in apps that allow it, so a default setting improves UX and reduces frustration a lot.

No files, tests, or entry points are named. Start by locating the existing disappearing-message controls for private and public chats and the settings flow for default expiration; done means a configurable default applies consistently when new DMs and public conversations are created.

Written by the indexing model from the issue text.