style: update card backdrop blur and fix shell layout

src/App.vue

@@ -45,10 +45,7 @@ const route = useRoute();
   position: sticky;
   top: 0;
   z-index: 10;
-  background: var(--surface);
-}
-
-.shell-main {
+}.shell-main {
   max-width: 1240px;
   width: 100%;
   margin: 0 auto;

src/assets/main.css

@@ -27,7 +27,9 @@ a:hover {
 }
 
 .surface-card {
-  background: var(--surface-strong);
+  background: rgba(255, 255, 255, 0.85);
+  backdrop-filter: blur(12px);
+  -webkit-backdrop-filter: blur(12px);
   border: 1px solid var(--border-soft);
   border-radius: 18px;
   box-shadow: var(--shadow-soft);