Fix checkout: show free-success when logged in; init Stripe without auth on paid plans

- revealPaymentForm: for free plan, show #free-success panel (was doing nothing,
  leaving page blank when user already had a Supabase session)
- checkExistingSession: for paid plans with no session, call initStripe immediately —
  auth is optional, the payment form shouldn't wait indefinitely
- Guard _formRevealed: prevent double-call from handleAuthRedirect + checkExistingSession

src/js/checkout-auth.el

@@ -29,12 +29,19 @@ fn main() -> Void {
     el.style.color = isError ? '#c0392b' : '#2ecc71';
   }
 
+  var _formRevealed = false;
   function revealPaymentForm(user) {
+    if (_formRevealed) return;
+    _formRevealed = true;
     if (user && user.id) { window._neuronSupaId = user.id; }
     var auth = document.getElementById('auth-section');
     if (auth) auth.style.display = 'none';
     var isFree = (window.NEURON_CFG || {}).plan === 'free';
-    if (!isFree) {
+    if (isFree) {
+      // Free plan: show the success panel (user is signed in or just signed up)
+      var freeSuccess = document.getElementById('free-success');
+      if (freeSuccess) freeSuccess.style.display = '';
+    } else {
       var payment = document.getElementById('payment-section');
       if (payment) payment.style.display = '';
     }
@@ -68,7 +75,16 @@ fn main() -> Void {
   function checkExistingSession() {
     initSupabase(function() {
       supabaseClient.auth.getUser().then(function(res) {
-        if (res.data && res.data.user) { revealPaymentForm(res.data.user); }
+        if (res.data && res.data.user) {
+          revealPaymentForm(res.data.user);
+        } else {
+          // No existing session — for paid plans, init Stripe immediately.
+          // Auth is optional on paid plans; the user can link their account later.
+          var isFree = (window.NEURON_CFG || {}).plan === 'free';
+          if (!isFree && typeof window.initStripe === 'function') {
+            window.initStripe('', '');
+          }
+        }
       });
     });
   }