EsLint v9 (#154)

* Upgrade ESLint and related configurations to version 9

- Update ESLint to version 9.19.0
- Migrate ESLint configurations to flat config format
- Remove deprecated ESLint config files
- Update package dependencies and configurations
- Simplify ESLint setup across packages
- Remove unnecessary ESLint config blocks from package.json files
- Improved CI caching with Turborepo tasks
- Removed duplicate styles

packages/billing/gateway/src/components/pricing-table.tsx

@@ -469,7 +469,7 @@ function DefaultCheckoutButton(
   });
 
   const linkHref =
-    props.plan.href ?? `${signUpPath}?${searchParams.toString()}` ?? '';
+    props.plan.href ?? `${signUpPath}?${searchParams.toString()}`;
 
   const label = props.plan.buttonLabel ?? 'common:getStartedWithPlan';