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-free.el

@@ -1,15 +1,18 @@
-// checkout-free.el -- Free plan: reveal payment section after auth completes.
-// Watches the auth-badge element; when it becomes visible, shows payment-section.
+// checkout-free.el -- Free plan: show success panel after auth completes.
+// Watches the auth-badge element; when it becomes visible, hides the auth
+// section and shows the free-success panel. No card required for free tier.
 // Compiled with: elc --target=js --bundle --minify --obfuscate
 
 fn main() -> Void {
     native_js("(function() {
-  var pay = document.getElementById('payment-section');
-  if (!pay) return;
+  var success = document.getElementById('free-success');
+  var auth = document.getElementById('auth-section');
+  if (!success) return;
   var timer = setInterval(function() {
     var badge = document.getElementById('auth-badge');
     if (badge && badge.offsetParent !== null) {
-      pay.style.display = '';
+      if (auth) auth.style.display = 'none';
+      success.style.display = '';
       clearInterval(timer);
     }
   }, 150);