Walkthrough slides keep one layout and zoom like demo videos

A walkthrough slide now has one fixed layout that the player scales to its width, so it looks the same in the narrow chat panel as when shown large. Press Z, click the magnifier, or click a playing slide to zoom the walkthrough to the size of the window, just like a demo video on a card. Because every slide has this one layout, Obeya checks each slide the moment the agent hands it over: a slide that does not fit, or whose narration only reads it out, goes back to the agent with what to change.