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/checkout.el

@@ -97,7 +97,7 @@ fn checkout_page(plan: String, pub_key: String) -> String {
       <div id=\"auth-section\" " + (if is_free { "" } else { "style=\"display:none;\"" }) + ">
         " + (if is_free { "
         <p class=\"label\" style=\"margin-bottom: 1.5rem; color: var(--navy);\">Create your account.</p>
-        <p class=\"checkout-auth-hint\" style=\"margin-bottom: 2rem;\">No charge today. Add your card to reserve your spot - you won&#39;t be billed until you upgrade.</p>
+        <p class=\"checkout-auth-hint\" style=\"margin-bottom: 2rem;\">No card required. Your account is free, forever.</p>
         " } else { "
         <p class=\"label\" style=\"margin-bottom: 1.25rem;\">Sign in (optional)</p>
         <p class=\"checkout-auth-hint\">Sign in to link this purchase to an existing account. Or skip and create one later - we'll match it to your email.</p>
@@ -135,6 +135,16 @@ fn checkout_page(plan: String, pub_key: String) -> String {
         </div>
       </div>
 
+      <!-- Free-tier success panel: shown after account creation, no card needed -->
+      " + (if is_free { "
+      <div id=\"free-success\" style=\"display:none; text-align:center; padding: 2.5rem 1rem;\">
+        <div style=\"font-size:2.5rem; margin-bottom:1.25rem;\">&#10003;</div>
+        <p class=\"label\" style=\"margin-bottom:.75rem; color:var(--navy);\">You&#39;re in.</p>
+        <p class=\"checkout-auth-hint\" style=\"margin-bottom:2rem;\">Your free account is ready. Download Neuron to get started.</p>
+        <a href=\"/marketplace\" class=\"checkout-submit\" style=\"display:inline-block; text-decoration:none; padding:.875rem 2rem;\">Go to your account &#8594;</a>
+      </div>
+      " } else { "" }) + "
+
       <!-- Payment form (visible immediately - no auth wall) -->
       <div id=\"payment-section\" " + (if is_free { "style=\"display:none;\"" } else { "" }) + ">
         <div id=\"auth-badge\" style=\"display:none; margin-bottom: 1.5rem;\"></div>