Export: record slide resolution in PNG and JPEG pictures

A default export is 2 pixels per point. Without a resolution, Preview
treats those pixels as 72 dpi and the picture opens at twice the slide's
size. PNG pHYs and JPEG JFIF density now say scale times 72, so the
default is 144 dpi. Screenshots stay untagged.

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/deckcraft-cli/src/main.rs

@@ -232,7 +232,7 @@ fn render_transition(args: &[String]) -> Result<(), String> {
             let name = format!("{stem}-t{t:.3}.png");
             p.with_file_name(name).to_string_lossy().into_owned()
         };
-        std::fs::write(&path, img.to_png()).map_err(|e| format!("{path}: {e}"))?;
+        std::fs::write(&path, img.to_png_at(render::dpi_for_scale(opts.scale))).map_err(|e| format!("{path}: {e}"))?;
         outln!("{path} ({}×{}) {} {} t={t}", img.width, img.height, tr.kind, tr.option);
     }
     Ok(())