fix: ad banner image uses faded edges instead of sharp borders

Apply radial-gradient mask for smooth vignette fade on all edges.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

public/js/modules/chess/scenes/game.js

@@ -893,7 +893,7 @@ async function loadAdBanner(el) {
     const wrapper = el.querySelector('#ad-banner-top');
     const content = el.querySelector('#ad-content');
     if (!wrapper || !content) return;
-    content.innerHTML = `<img src="${res.image_url}" style="width:100%;height:100%;object-fit:cover;border-radius:10px;">`;
+    content.innerHTML = `<img src="${res.image_url}" style="width:100%;height:100%;object-fit:cover;border-radius:12px;-webkit-mask-image:radial-gradient(ellipse 90% 85% at center,black 60%,transparent 100%);mask-image:radial-gradient(ellipse 90% 85% at center,black 60%,transparent 100%);">`;
     wrapper.style.display = 'flex';
     window.__adClick = () => {
       if (res.click_url) window.open(res.click_url, '_blank');