Update Polar SDK checkout API for v0.42.5 compatibility (#3360)

* Initial plan

* Update Polar SDK checkout API from custom.create to create with products array

Co-authored-by: RobbieTheWagner <[email protected]>

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: RobbieTheWagner <[email protected]>

Author: Copilot

landing/src/actions/index.ts

@@ -17,8 +17,8 @@ export const server = {
       let result;
 
       try {
-        result = await polar.checkouts.custom.create({
-          productPriceId,
+        result = await polar.checkouts.create({
+          products: [productPriceId],
           successUrl: 'https://docs.shepherdjs.dev'
         });
       } catch (error) {