fix(auth): wire up dashboard auth guard as Next.js proxy (#227)

Dashboard routes were accessible without authentication because
dashboardGuard.js was not connected to Next.js middleware pipeline.

Created src/proxy.js to export the auth guard using Next.js 16
proxy convention. Now /dashboard routes properly redirect to
/login when auth_token cookie is missing or invalid.

src/proxy.js

@@ -0,0 +1,5 @@
+export { proxy } from "./dashboardGuard";
+
+export const config = {
+  matcher: ["/", "/dashboard/:path*"],
+};