Center Real-Time Sync slide content vertically

Add justify-content: center to vertically center the diagram and text
in the available space below the title.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: sanity

hugo-site/static/slides/hard-problems/real-time-sync.html

@@ -6,6 +6,7 @@ <h2 style="flex-shrink: 0; margin-bottom: 0.3em;">Real-Time Sync</h2>
       flex: 1;
       display: flex;
       align-items: center;
+      justify-content: center;
       gap: 1.5em;
       min-height: 0;
     }
@@ -32,6 +33,7 @@ <h2 style="flex-shrink: 0; margin-bottom: 0.3em;">Real-Time Sync</h2>
     @media (max-width: 1400px) {
       .realtime-content {
         flex-direction: column;
+        justify-content: center;
       }
       .realtime-diagram {
         flex: 0 0 auto;