Fix input/message alignment on wide screens

- Add max-width 800px to message list and input area
- Center align both components for consistent layout
- Wrap input in container for full-width background

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

frontend/src/components/MessageList.css

@@ -5,6 +5,10 @@
   display: flex;
   flex-direction: column;
   gap: 12px;
+  max-width: 800px;
+  margin: 0 auto;
+  width: 100%;
+  box-sizing: border-box;
 }
 
 .empty-state {