feat(ui): add smooth collapse animation for subscriptions list

Replace display toggle with CSS transitions for subscriptions list collapse

css/style.css

@@ -200,6 +200,10 @@ body {
     text-transform: uppercase;
 }
 
+.nav-section-title i {
+    transition: transform 0.3s ease;
+}
+
 .nav-btn {
     display: flex;
     align-items: center;
@@ -231,6 +235,16 @@ body {
 
 .subscriptions-list {
     padding: 0 12px;
+    max-height: 500px;
+    overflow: hidden;
+    transition: max-height 0.3s ease, opacity 0.3s ease;
+    opacity: 1;
+}
+
+.subscriptions-list.collapsed {
+    max-height: 0;
+    opacity: 0;
+    padding: 0 12px;
 }
 
 .subscription-item {