fix(contacts): Fix CSS specificity conflict limiting list height

Problem: #existingList ID selector had higher specificity than
.contacts-list-fullscreen class, and max-height values were limiting
the list height regardless of the class height setting.

Changes:
- Desktop: #existingList max-height calc(100vh - 400px) → calc(100vh - 240px)
- Mobile: #existingList max-height calc(100vh - 450px) → calc(100vh - 280px)

This should now properly utilize available screen space.

app/templates/contacts_base.html

@@ -182,7 +182,7 @@
             overflow-y: auto;
             overflow-x: hidden;
             -webkit-overflow-scrolling: touch;
-            max-height: calc(100vh - 400px);
+            max-height: calc(100vh - 240px);
             min-height: 300px;
         }
 
@@ -328,7 +328,7 @@
         /* Mobile responsiveness */
         @media (max-width: 768px) {
             #existingList {
-                max-height: calc(100vh - 450px);
+                max-height: calc(100vh - 280px);
             }
 
             .contacts-list-fullscreen {