Align input field width with message bubbles

- Set input-wrapper to 70% width to match message bubble max-width
- Remove unnecessary max-width constraints from containers

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

frontend/src/components/MessageList.css

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