feat: add new subscription tier and update credit configurations

- Introduced a new "max" subscription tier with associated monthly credits and top-up limits.
- Updated existing subscription tiers' monthly credits and top-up limits for "starter", "pro", and "business".
- Enhanced credit display and overview components to reflect the new tier and its attributes.
- Integrated Polar authentication features for improved subscription management and credit handling.

lib/auth-client.ts

@@ -1,7 +1,8 @@
 import { createAuthClient } from "better-auth/react";
 import { convexClient } from "@convex-dev/better-auth/client/plugins";
+import { polarClient } from "@polar-sh/better-auth/client";
 
 // Next.js: kein crossDomainClient nötig (same-origin via API Route Proxy)
 export const authClient = createAuthClient({
-  plugins: [convexClient()],
+  plugins: [convexClient(), polarClient()],
 });