ui: keep the scroll viewport mask visible (alpha 0 clipped all content)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/Decentralized/PledgerSkin.cs

@@ -197,7 +197,8 @@ namespace Decentralized
                 if (scroll.viewport != null)
                 {
                     var vpImg = scroll.viewport.GetComponent<Image>();
-                    if (vpImg != null) vpImg.color = Transparent;
+                    // 视口的图是 Mask 的模板：alpha 为 0 会把整个内容裁掉，只能压到几乎看不见
+                    if (vpImg != null) vpImg.color = new Color(0f, 0f, 0f, 0.004f);
                 }
                 if (scroll.verticalScrollbar != null)
                 {