Streamline "Accept Realtime Battle" input

[dmcomm]

I proposed on Discord:

• move device selection above invite code
• changing device selection clears invite code

(There still needs to be an error message if user enters the same invite code again.)

[dmcomm]

Update helps a lot, but we can get it into a bad state by changing device (optionally changing it back) and then trying to reuse the invite code.