feat: allow premium+ users to manage costs

- costs routes drop requireAdmin, keep auth + costs feature check
- useAuth exposes hasFeature(feature) based on entitlements
- tool-detail costs section gated by hasFeature('costs') instead of isAdmin

artifacts/toolrate/src/hooks/use-auth.ts

@@ -22,6 +22,12 @@ export function useAuth() {
   const isLocalMode = authMode?.mode === "local";
   const tier = isAuthenticated ? user?.tier ?? "free" : "free";
 
+  function hasFeature(feature: string): boolean {
+    if (!isAuthenticated) return false;
+    if (user?.role === "admin") return true;
+    return !!user?.entitlements?.includes(feature);
+  }
+
   function login(returnTo?: string) {
     if (isLocalMode) {
       const path = returnTo
@@ -47,6 +53,7 @@ export function useAuth() {
     isAdmin,
     isLocalMode,
     tier,
+    hasFeature,
     login,
     logout,
   };