Don't let users send unrecognised slash commands

### Your use case

#### What would you like to do?

When I type something unrecognised like /foo I don't expect it to be sent into the room verbatim if it's not recognised - this is a recipe for accidentally leaking commands into the room if you typo them or misremember them.

#### Why would you like to do it?

Stop looking like a twit when I typo/forget a slash command on iOS

#### How would you like to achieve it?

Chuck up a "unrecogised slash command" alert rather than blindly sending it into the room.

### Have you considered any alternatives?

Even better would be to autocomplete slash commands like on Web, but that feels like a separate issue.

### Additional context

_No response_