fix(chat): replace h-full with flex-1 min-h-0 for ChatArea container

h-full = 100% of parent height, but TopBar already occupies 56px above.
This caused ChatArea to overflow by 56px, pushing the input box below
the visible viewport. flex-1 + min-h-0 correctly fills remaining space
in the flex column layout.

desktop/src/components/ChatArea.tsx

@@ -266,7 +266,7 @@ export function ChatArea() {
   );
 
   return (
-    <div className="relative h-full">
+    <div className="relative flex-1 min-h-0">
     {/* Generation progress overlay */}
     <AnimatePresence>
       {generating && (