Can I play Janna, Savior in a Showdown when I am the attacker?

Yes, you can play Janna, Savior in a showdown while you are the attacker, provided you are playing her to a battlefield you control.

Reasoning

• Reaction Keyword: Janna, Savior has the [Reaction] keyword, which explicitly allows her to be played at any time, including during the Closed state of a showdown.
• Control Restriction: According to RiftJudge FAQ #9449, units must be played to a location you control. As the attacker in a showdown, you do not control the battlefield where the combat is taking place. However, the [Reaction] text on Janna, Savior allows her to be played "to a battlefield you control."

Therefore, as long as you are playing Janna, Savior to a battlefield you already control (a friendly battlefield), this is a legal move during the showdown. You cannot play her to the battlefield where the combat is currently taking place if that battlefield is contested and controlled by your opponent.