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-stripe.el

@@ -31,8 +31,13 @@ fn main() -> Void {
     if (spinner) spinner.style.display = loading ? ''     : 'none';
   }
 
+  // Free plan has no payment form — bail out entirely.
+  if (str_eq(PLAN, 'free')) return;
+
   window._neuronMode = 'payment';
   var paymentEl = null;
+  var userEmail  = '';
+  var userName   = '';
 
   function appearance() {
     return {
@@ -80,7 +85,7 @@ fn main() -> Void {
     return fetch('/api/payment-intent', {
       method: 'POST',
       headers: {'Content-Type': 'application/json'},
-      body: JSON.stringify({ plan: PLAN, timing: timing })
+      body: JSON.stringify({ plan: PLAN, timing: timing, email: userEmail, name: userName })
     })
     .then(function(r) { return r.json(); })
     .then(function(data) {
@@ -117,11 +122,17 @@ fn main() -> Void {
     });
   }
 
-  fetchAndMount();
-  var tNow   = document.getElementById('timing-now');
-  var tLater = document.getElementById('timing-later');
-  if (tNow)   tNow.addEventListener('change',   fetchAndMount);
-  if (tLater) tLater.addEventListener('change', fetchAndMount);
+  // Don't init Stripe at page load — wait for auth.
+  // checkout-auth.el calls window.initStripe(email, name) after sign-in.
+  window.initStripe = function(email, name) {
+    userEmail = email || '';
+    userName  = name  || '';
+    fetchAndMount();
+    var tNow   = document.getElementById('timing-now');
+    var tLater = document.getElementById('timing-later');
+    if (tNow)   tNow.addEventListener('change',   fetchAndMount);
+    if (tLater) tLater.addEventListener('change', fetchAndMount);
+  };
 
   var form = document.getElementById('payment-form');
   if (form) form.addEventListener('submit', async function(e) {