Fix: show banner full height + reliable dropdown

- Increased brand-card-gap min-height to 500px (desktop) so
  the full background image is visible (509px at 1200px grid width)
- Responsive: 400px at 960px, 240px at 600px
- Dropped grid-row spanning approach; back to CSS background-image
  on .brands-grid with background-size: 100% auto
- Dropdown sets backgroundImage directly on .brands-grid via
  style.backgroundImage — no setTimeout, no opacity animation,
  no async timing issues

style.css

@@ -381,18 +381,10 @@ body::before {
     grid-template-columns: 1fr 1fr;
     max-width: 1200px;
     margin: 0 auto;
-}
-
-/* Full-width banner spanning both columns in the middle row */
-.brands-banner-img {
-    grid-column: 1 / -1;
-    grid-row: 2;
-    width: 100%;
-    height: auto;
-    display: block;
-    z-index: 0;
-    pointer-events: none;
-    transition: opacity 0.3s ease;
+    background-image: url("asset/banner/1.png");
+    background-size: 100% auto;
+    background-position: center;
+    background-repeat: no-repeat;
 }
 
 .brand-card {
@@ -401,19 +393,15 @@ body::before {
     display: flex;
     flex-direction: column;
     justify-content: space-between;
-    grid-row: 1 / 4;
     text-decoration: none;
     cursor: pointer;
-    z-index: 1;
 }
 
 .brand-card--fine {
-    grid-column: 1;
     border-right: 1px solid var(--border);
 }
 
 .brand-card--lacueor {
-    grid-column: 2;
 }
 
 .brand-card-header {
@@ -430,6 +418,11 @@ body::before {
     background: var(--lacueor-dark);
 }
 
+.brand-card-gap {
+    flex: 1;
+    min-height: 500px;
+}
+
 .brand-card-footer {
     flex-shrink: 0;
     padding: 20px 52px 40px;
@@ -444,7 +437,7 @@ body::before {
     background: var(--lacueor-dark);
 }
 
-.brand-card-spacer {
+.brands-banner-img {
     display: none;
 }
 
@@ -1008,6 +1001,10 @@ footer {
         padding: 24px 28px 16px;
     }
 
+    .brand-card-gap {
+        min-height: 400px;
+    }
+
     .brand-card-footer {
         padding: 16px 28px 32px;
     }
@@ -1060,6 +1057,10 @@ footer {
         padding: 20px 24px 14px;
     }
 
+    .brand-card-gap {
+        min-height: 240px;
+    }
+
     .brand-card-footer {
         padding: 14px 24px 28px;
     }