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/topup-calculator.ts

@@ -0,0 +1,14 @@
+export function calculateCustomTopup(euroAmount: number): {
+  credits: number;
+  bonusRate: number;
+} {
+  const bonusRate =
+    euroAmount >= 100 ? 0.13 :
+    euroAmount >= 50 ? 0.10 :
+    euroAmount >= 20 ? 0.06 :
+    euroAmount >= 10 ? 0.03 : 0;
+
+  const netAmount = euroAmount / 1.19;
+  const credits = Math.floor((netAmount * 0.70 * (1 + bonusRate)) / 0.01);
+  return { credits, bonusRate };
+}