fix: 电商页面 KeepAlive 容器使用绝对定位铺满视口，解决底部留白问题

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/App.tsx

@@ -1237,7 +1237,7 @@ function App() {
 
         {/* KeepAlive: EcommercePage stays mounted once visited */}
         {ecommerceEverMounted && (
-          <div style={{ display: isEcommerceActive ? undefined : "none" }}>
+          <div style={{ display: isEcommerceActive ? undefined : "none", position: "absolute", inset: 0, zIndex: 1 }}>
             <EcommercePage
               projects={projects}
               isAuthenticated={Boolean(session)}

src/styles/shell/app-shell.css

@@ -559,6 +559,7 @@
 }
 
 .web-shell__page {
+  position: relative;
   flex: 1;
   min-height: 0;
   overflow: auto;