feat: Multi-session support with tabs, split view, and Mochi integration

- Add SessionContext for central state management
- Add TabBar component for session tabs
- Add SplitLayout for side-by-side session viewing
- Add ChatPanel wrapper component
- Refactor ChatInput to uncontrolled input for performance
- Add SCP file transfer for SSH hosts (Mochi)
- Fix stats undefined crash on session restore
- Store host info in sessions for upload routing

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

frontend/src/components/MessageList.jsx

@@ -72,7 +72,7 @@ function SystemHints({ reminders, inline = false }) {
   );
 }
 
-export function MessageList({ messages, isProcessing }) {
+export const MessageList = memo(function MessageList({ messages, isProcessing }) {
   const containerRef = useRef(null);
   const messagesEndRef = useRef(null);
   const [showScrollButton, setShowScrollButton] = useState(false);
@@ -215,7 +215,7 @@ export function MessageList({ messages, isProcessing }) {
       )}
     </div>
   );
-}
+});
 
 const Message = memo(function Message({ message }) {
   const { type, content, tool, input, timestamp, toolUseId, attachments } = message;