fix: free tier checkout - full Stripe form with SetupIntent (card saved, no charge)

src/main.el

@@ -579,8 +579,29 @@ fn handle_request(method: String, path: String, body: String) -> String {
         if str_contains(body, "\"professional\"") {
             plan = "professional"
         }
+        if str_contains(body, "\"free\"") {
+            plan = "free"
+        }
         let timing: String = json_get_string(body, "timing")
         if str_eq(timing, "") { let timing = "now" }
+        // Free tier: SetupIntent — save card details without charging.
+        // Card is stored on a Stripe Customer; billing begins only if the
+        // user later upgrades to a paid plan.
+        if str_eq(plan, "free") {
+            let si_body: String = "automatic_payment_methods[enabled]=true"
+                + "&usage=off_session"
+                + "&metadata[plan]=free"
+            let auth_header: String = "Bearer " + stripe_key
+            let si_resp: String = http_post_form_auth(
+                "https://api.stripe.com/v1/setup_intents",
+                si_body,
+                auth_header)
+            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
+        }
         // Hard cap: block founding checkouts when 1,000 spots are filled
         if str_eq(plan, "founding") {
             let current_sold: Int = get_sold()