refactor(chat): extract ai disclosure into dedicated banner component

- Move AiDisclosure and EmptyState logic into a new AiDisclosureBanner component
- Remove isGuest prop from ChatArea as it's no longer needed there
- Update styling with explicit color values for better visibility

src/app/chat/components/chat-area.module.css

@@ -22,6 +22,7 @@
   border-radius: var(--radius-md, 8px);
   padding: var(--spacing-1, 4px) var(--spacing-3, 12px);
   font-size: var(--font-size-xs, 11px);
+  color: #000;
   opacity: 0.75;
 }
 
@@ -31,7 +32,7 @@
   border-radius: var(--radius-md, 8px);
   padding: var(--spacing-2, 8px) var(--spacing-4, 16px);
   font-size: var(--font-size-sm, 12px);
-  opacity: 0.75;
+  color: #8c8b8b;
   text-align: center;
 }