Fix free plan checkout: use SetupIntent instead of $0 PaymentIntent

Stripe rejects amount=0 PaymentIntents. Free plan age verification should
use a SetupIntent (no charge, saves payment method). The JS already handles
setup_mode:true by calling stripe.confirmSetup instead of confirmPayment.
Mirrors the existing professional-later SetupIntent path.

src/main.el

@@ -699,21 +699,24 @@ fn handle_request_inner(method: String, path: String, headers: Map, body: String
             }
         }
 
-        // Free tier: $0 PaymentIntent for age verification (18+ requirement).
+        // Free tier: SetupIntent for age verification (18+ requirement).
-        // Verifies card is valid. No charge, no capture.
+        // Verifies card is valid and saves it. No charge, no capture.
-        // Note: setup_future_usage cannot be used with amount=0.
+        // $0 PaymentIntents are rejected by Stripe; SetupIntent is the correct tool.
         if str_eq(plan, "free") {
-            let free_pi_body: String = "amount=0"
+            let si_body: String = "automatic_payment_methods[enabled]=true"
-                + "&currency=usd"
+                + "&usage=off_session"
-                + "&payment_method_types[]=card"
                 + "&metadata[plan]=free"
                 + "&metadata[purpose]=age_verification"
-            let free_pi_body = if !str_eq(pi_cus_id, "") { free_pi_body + "&customer=" + pi_cus_id } else { free_pi_body }
+            let si_body = if !str_eq(pi_cus_id, "") { si_body + "&customer=" + pi_cus_id } else { si_body }
-            let free_pi_resp: String = http_post_form_auth(
+            let si_resp: String = http_post_form_auth(
-                "https://api.stripe.com/v1/payment_intents",
+                "https://api.stripe.com/v1/setup_intents",
-                free_pi_body,
+                si_body,
                 auth_header)
-            return free_pi_resp
+            if str_starts_with(si_resp, "{") {
+                let inner: String = str_slice(si_resp, 1, str_len(si_resp))
+                return "{\"setup_mode\":true,\"plan\":\"free\"," + inner
+            }
+            return si_resp
         }
 
         // Setup-mode path: save payment method, do not charge. Only valid