chat: prevent input overflow offscreen
chat: prevent input overflow offscreen

Fixes urbit/landscape#632

urbit/urbit,urbit/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,urbit/urbit