Fix free tier checkout and Stripe duplicate customers

Free tier:
- checkout-stripe.el bails out immediately for plan=free (no Stripe init)
- checkout-auth.el skips payment section reveal and initStripe for free plan
- checkout-free.el shows #free-success panel after auth (no card ever shown)
- /api/payment-intent returns early for free plan — no Stripe call

Stripe dedup (all paid plans):
- Stripe init now deferred to window.initStripe(email, name), called by
  checkout-auth.el after sign-in — email is known before intent is created
- /api/payment-intent finds-or-creates Stripe Customer by email before
  creating the PaymentIntent/SetupIntent and attaches customer upfront
- Eliminates the window between intent creation and /api/link-customer
  that was producing duplicate guest customers

src/js/checkout-auth.el

@@ -33,8 +33,11 @@ fn main() -> Void {
     if (user && user.id) { window._neuronSupaId = user.id; }
     var auth = document.getElementById('auth-section');
     if (auth) auth.style.display = 'none';
-    var payment = document.getElementById('payment-section');
-    if (payment) payment.style.display = '';
+    var isFree = (window.NEURON_CFG || {}).plan === 'free';
+    if (!isFree) {
+      var payment = document.getElementById('payment-section');
+      if (payment) payment.style.display = '';
+    }
 
     if (user) {
       var badge = document.getElementById('auth-badge');
@@ -55,9 +58,11 @@ fn main() -> Void {
       if (emailEl) emailEl.value = user.email;
     }
 
-    var userEmail = user ? (user.email || '') : '';
-    var userName  = user ? ((user.user_metadata && user.user_metadata.full_name) || '') : '';
-    if (typeof initStripe === 'function') initStripe(userEmail, userName);
+    if (!isFree) {
+      var userEmail = user ? (user.email || '') : '';
+      var userName  = user ? ((user.user_metadata && user.user_metadata.full_name) || '') : '';
+      if (typeof window.initStripe === 'function') window.initStripe(userEmail, userName);
+    }
   }
 
   function checkExistingSession() {