feat(web): scaffold Next.js 16 dashboard with design system and auth client (#82)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

apps/web/src/app/(dashboard)/chat/page.tsx

@@ -0,0 +1,8 @@
+export default function ChatPage(): React.ReactElement {
+  return (
+    <div>
+      <h1 className="text-2xl font-semibold">Chat</h1>
+      <p className="mt-2 text-text-secondary">Conversations will appear here.</p>
+    </div>
+  );
+}