fix(contacts): Use fixed height instead of max-height to prevent page scroll

Problem: List was too tall causing the entire page to scroll,
hiding navigation buttons at the top when scrolling to bottom.

Solution:
- Changed from max-height to height for #existingList
- Desktop: calc(100vh - 260px) - slightly smaller to fit all elements
- Mobile: calc(100vh - 300px) - adjusted for mobile layout

Now only the contact list scrolls internally, while navigation
buttons remain visible at the top.

app/templates/contacts_base.html

@@ -182,7 +182,7 @@
             overflow-y: auto;
             overflow-x: hidden;
             -webkit-overflow-scrolling: touch;
-            max-height: calc(100vh - 240px);
+            height: calc(100vh - 260px);
             min-height: 300px;
         }
 
@@ -328,11 +328,11 @@
         /* Mobile responsiveness */
         @media (max-width: 768px) {
             #existingList {
-                max-height: calc(100vh - 280px);
+                height: calc(100vh - 300px);
             }
 
             .contacts-list-fullscreen {
-                height: calc(100vh - 280px);
+                height: calc(100vh - 300px);
             }
 
             .filter-sort-toolbar {