Apply paint opacity to CSS gradient stops in code export (#1750)

Found while testing crates/engine/src/dev/codegen/css.rs:121 (css_decls, the PaintKind::Gradient arm calling css_gradient at :255)

Co-authored-by: Claude Haiku 5.5 <noreply@anthropic.com>

crates/engine/src/dev/codegen.rs

@@ -494,6 +494,18 @@ mod tests {
         f
     }
 
+    #[test]
+    fn css_gradient_folds_in_paint_opacity() {
+        let mut s = Session::new();
+        let f = id_of(&s.execute("frame.create", &json!({"x": 0, "y": 0, "width": 200, "height": 100})).unwrap()["id"]).unwrap();
+        let mut p = figcraft_color::Paint::gradient(figcraft_color::Gradient::default_for(figcraft_color::GradientKind::Linear, Color::BLACK));
+        p.opacity = 0.5;
+        s.doc_mut().unwrap().get_mut(f).unwrap().fills = vec![p];
+        let css = code_for(s.doc().unwrap(), f, CodeLang::Css);
+        assert!(css.contains("rgba(0, 0, 0, 0.50) 0%"), "{css}");
+        assert!(!css.contains("#000000 0%"), "{css}");
+    }
+
     #[test]
     fn swiftui_and_compose_for_an_auto_layout_card() {
         let mut s = Session::new();

crates/engine/src/dev/codegen/css.rs

@@ -120,7 +120,7 @@ pub(crate) fn css_decls(doc: &Document, n: &Node, t: &mut Tokens, hug: (bool, bo
                 // A variable binds the first paint's colour.
                 put(key, if i == 0 { tok(doc, n, &["fill"], c, t) } else { c });
             }
-            PaintKind::Gradient(g) => put("background", css_gradient(g, n.width, n.height)),
+            PaintKind::Gradient(g) => put("background", css_gradient(g, n.width, n.height, p.opacity)),
             PaintKind::Image(_) => put("background", "url(image.png) center / cover".into()),
             PaintKind::Pattern(_) => put("background", "url(pattern.png) repeat".into()),
         }
@@ -307,8 +307,13 @@ fn decoration_decls(s: &figcraft_doc::TextStyle, put: &mut impl FnMut(&str, Stri
 /// A gradient fill on a `w`×`h` box as a CSS background: linear along its handles' direction
 /// (CSS angles: 0deg up, 90deg right), radial with its handles' radii, angular as a conic
 /// gradient from its start direction.
-fn css_gradient(g: &figcraft_color::Gradient, w: f64, h: f64) -> String {
-    let stops = g.sorted_stops().iter().map(|s| format!("{} {}%", s.color.to_css(), (s.position * 100.0).round())).collect::<Vec<_>>().join(", ");
+fn css_gradient(g: &figcraft_color::Gradient, w: f64, h: f64, opacity: f32) -> String {
+    let stops = g
+        .sorted_stops()
+        .iter()
+        .map(|s| format!("{} {}%", s.color.with_alpha(s.color.a * opacity).to_css(), (s.position * 100.0).round()))
+        .collect::<Vec<_>>()
+        .join(", ");
     let (dx, dy) = ((g.to.0 - g.from.0) * w, (g.to.1 - g.from.1) * h);
     let angle = if dx == 0.0 && dy == 0.0 { 180.0 } else { dx.atan2(-dy).to_degrees().rem_euclid(360.0) };
     let pct = |v: f64| format!("{}%", num(v * 100.0));