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/MessageInput.css

@@ -2,8 +2,10 @@
   display: flex;
   gap: 12px;
   padding: 16px;
-  background: #16213e;
-  border-top: 1px solid #0f3460;
+  max-width: 800px;
+  margin: 0 auto;
+  width: 100%;
+  box-sizing: border-box;
 }
 
 .message-input .input-wrapper {