fix(chat): input box cut off — add flex-shrink-0 to prevent flex compression

The input area at the bottom of the chat panel lacked flex-shrink-0,
causing the flex column layout to compress it when message content filled
the Conversation area. This made the textarea only partially visible and
the scrollbar unable to reach the bottom of the input area.

desktop/src/components/ChatArea.tsx

@@ -481,7 +481,7 @@ export function ChatArea() {
       </Conversation>
 
       {/* Input */}
-      <div className="p-4 bg-white dark:bg-gray-900">
+      <div className="flex-shrink-0 p-4 bg-white dark:bg-gray-900">
         <div className="max-w-4xl mx-auto">
           {/* Suggestion chips */}
           {!isStreaming && suggestions.length > 0 && !messages.some(m => m.error) && (