checkout: re-add link-customer + redirect to /account, fix success/account routes

Restores the /api/link-customer endpoint that was lost in the stash. It
runs right before stripe.confirmPayment() and:
  - searches Stripe for an existing Customer by email
  - creates one if missing (URL-encodes + and @ so Gmail aliases work)
  - attaches the Customer to the live PaymentIntent
  - upserts the Supabase waitlist row with stripe_customer_id + plan

Stripe locks customer on a Charge once set, so the webhook handler is
the second-line backstop for any race where confirmPayment beats the
link call.

Also: change return_url from /marketplace/success to /account?welcome=1
so buyers land where they need to be, and switch /marketplace/success
and /account from str_eq to str_starts_with so Stripe-appended query
strings (?payment_intent=...&redirect_status=succeeded) don't 404.

src/checkout.el

@@ -703,6 +703,9 @@ fn checkout_page(plan: String, pub_key: String) -> String {
       return;
     }
 
+    // Capture the PI id so we can attach a Customer at submit time
+    window._neuronPiId = data.id || (data.client_secret ? data.client_secret.split('_secret_')[0] : '');
+
     waitForStripe(function() {
       stripe = Stripe(STRIPE_PK);
 
@@ -815,10 +818,24 @@ fn checkout_page(plan: String, pub_key: String) -> String {
     setLoading(true);
     document.getElementById('payment-message').style.display = 'none';
 
+    // Link a Stripe Customer to this PaymentIntent and to the Supabase waitlist row
+    // (so the buyer shows up as a named customer, not Guest, and the account page
+    //  can find their plan via stripe_customer_id). Non-blocking - if it fails, the
+    //  webhook still links them server-side after payment_intent.succeeded fires.
+    if (window._neuronPiId) {
+      try {
+        await fetch('/api/link-customer', {
+          method: 'POST',
+          headers: { 'Content-Type': 'application/json' },
+          body: JSON.stringify({ pi_id: window._neuronPiId, email: email, name: name, plan: PLAN })
+        });
+      } catch(e) { /* non-blocking */ }
+    }
+
     var result = await stripe.confirmPayment({
       elements: elements,
       confirmParams: {
-        return_url: window.location.origin + '/marketplace/success',
+        return_url: window.location.origin + '/account?welcome=1',
         payment_method_data: {
           billing_details: { name: name, email: email }
         },