Free plan: use $0 PaymentIntent instead of SetupIntent for age verification

All plans now use the same payment intent flow. Free creates a $0 PI
with payment_method_types[]=card and setup_future_usage=off_session.
No charge, card saved. Removes setup_mode=true for free plan.

Fix submit button label: show 'Verify age & get started' for free
instead of 'Complete purchase'. Retire checkout-free.el.

src/main.el

@@ -697,23 +697,21 @@ fn handle_request_inner(method: String, path: String, headers: Map, body: String
             }
         }
 
-        // Free tier: creates a SetupIntent for age verification (18+ requirement).
-        // No charge — but the user must provide a valid payment method.
+        // Free tier: $0 PaymentIntent for age verification (18+ requirement).
+        // Card is verified and saved (setup_future_usage=off_session). No charge.
         if str_eq(plan, "free") {
-            let free_si_body: String = "automatic_payment_methods[enabled]=true"
-                + "&usage=off_session"
+            let free_pi_body: String = "amount=0"
+                + "&currency=usd"
+                + "&payment_method_types[]=card"
+                + "&setup_future_usage=off_session"
                 + "&metadata[plan]=free"
                 + "&metadata[purpose]=age_verification"
-            let free_si_body = if !str_eq(pi_cus_id, "") { free_si_body + "&customer=" + pi_cus_id } else { free_si_body }
-            let free_si_resp: String = http_post_form_auth(
-                "https://api.stripe.com/v1/setup_intents",
-                free_si_body,
+            let free_pi_body = if !str_eq(pi_cus_id, "") { free_pi_body + "&customer=" + pi_cus_id } else { free_pi_body }
+            let free_pi_resp: String = http_post_form_auth(
+                "https://api.stripe.com/v1/payment_intents",
+                free_pi_body,
                 auth_header)
-            if str_starts_with(free_si_resp, "{") {
-                let inner: String = str_slice(free_si_resp, 1, str_len(free_si_resp))
-                return "{\"setup_mode\":true,\"plan\":\"free\"," + inner
-            }
-            return free_si_resp
+            return free_pi_resp
         }
 
         // Setup-mode path: save payment method, do not charge. Only valid