Cleaner chat actions, delete-chat, and sign-out

In a chat, the actions on a held message now trigger when you lift your finger, so you can slide away to cancel. Ending a conversation is clearer — a plain Delete chat with a short explanation and a confirmation — and signing out now takes a deliberate press-and-hold. In the browser, the message box rests right on the keyboard with no gap, and a button brings you back to the newest messages.