fix: cast role string to UserRole enum in auth.ts

Fixes TypeScript build error where JWT claims role (string) was
assigned to User.role (UserRole enum).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/lib/auth.ts

@@ -1,3 +1,4 @@
+import { UserRole } from "./types";
 import type { User, LoginResponse } from "./types";
 
 const TOKEN_KEY = "amazon_tc_token";
@@ -54,7 +55,7 @@ export function setAuth(response: LoginResponse): void {
       id: (claims.sub as string) || "",
       email: (claims.email as string) || "",
       name: (claims.name as string) || "",
-      role: ((claims.role as string) || "").toUpperCase(),
+      role: ((claims.role as string) || "").toUpperCase() as UserRole,
       is_active: true,
       created_at: new Date().toISOString(),
     };