fix(auth): enable Clerk middleware and make /sign-in public

frontend/src/middleware.ts

@@ -0,0 +1,2 @@
+export { default } from "./proxy";
+export { config } from "./proxy";

frontend/src/proxy.ts

@@ -9,7 +9,7 @@ const isClerkEnabled = () =>
   );
 
 // Public routes must include Clerk sign-in paths to avoid redirect loops.
-const isPublicRoute = createRouteMatcher(["/sign-in(.*)"]); 
+const isPublicRoute = createRouteMatcher(["/sign-in(.*)"]);
 
 export default isClerkEnabled()
   ? clerkMiddleware((auth, req) => {