fix(checkout): split checkout_page into helpers to avoid single-function OOM in elc --emit-header

Extract nav and style blocks into checkout_nav_html() and checkout_style_html()
so the compiler processes each template in isolation rather than one 490-line
function with mixed HTML template AST and BinOp string concat.

src/checkout.el

@@ -11,6 +11,17 @@
 //   4. User fills name, email, card - submits
 //   5. stripe.confirmPayment() → redirects to /marketplace/success
 
+fn checkout_nav_html() -> String {
+    <nav id="nav">
+  <div class="nav-inner">
+    <a href="/" class="nav-logo" aria-label="Neuron home"><img src="/assets/brand/neuron-wordmark-on-light.png" srcset="/assets/brand/neuron-wordmark-on-light@2x.png 2x" alt="Neuron" height="28"></a>
+    <div class="nav-links">
+      <a href="/" class="nav-link">&#8592; Back</a>
+    </div>
+  </div>
+</nav>
+}
+
 fn checkout_page(plan: String, pub_key: String) -> String {
     let is_founding: Bool = str_eq(plan, "founding")
     let is_free: Bool = str_eq(plan, "free")
@@ -53,17 +64,9 @@ fn checkout_page(plan: String, pub_key: String) -> String {
          <li>2 devices included</li>"
     } }
 
-    return
+    let nav_html: String = checkout_nav_html()
-<nav id="nav">
-  <div class="nav-inner">
-    <a href="/" class="nav-logo" aria-label="Neuron home"><img src="/assets/brand/neuron-wordmark-on-light.png" srcset="/assets/brand/neuron-wordmark-on-light@2x.png 2x" alt="Neuron" height="28"></a>
-    <div class="nav-links">
-      <a href="/" class="nav-link">&#8592; Back</a>
-    </div>
-  </div>
-</nav>
 
-<main style="min-height: 100vh; padding: clamp(6rem, 14vh, 9rem) 2rem 4rem;">
+    let main_html: String = <main style="min-height: 100vh; padding: clamp(6rem, 14vh, 9rem) 2rem 4rem;">
   <div class="checkout-shell">
 
     <!-- Left: order summary -->
@@ -236,23 +239,14 @@ fn checkout_page(plan: String, pub_key: String) -> String {
   </div>
 </main>
 
-<!-- Supabase JS -->
+    let style_html: String = checkout_style_html()
-<script src="https://cdn.jsdelivr.net/npm/@supabase/supabase-js@2/dist/umd/supabase.js"></script>
-<!-- Stripe.js -->
-<script src="https://js.stripe.com/v3/" async></script>
 
-<style>
+    let free_init_script: String = if is_free { "<script>document.addEventListener('DOMContentLoaded',function(){window.neuronCheckoutFree&&window.neuronCheckoutFree()});</script>" } else { "" }
-.checkout-shell {
+    return nav_html + main_html + "<!-- Supabase JS --><script src=\"https://cdn.jsdelivr.net/npm/@supabase/supabase-js@2/dist/umd/supabase.js\"></script><!-- Stripe.js --><script src=\"https://js.stripe.com/v3/\" async></script>" + style_html + "<script src=\"/js/checkout-auth.js\" defer></script><script>window.NEURON_CFG=window.NEURON_CFG||{};window.NEURON_CFG.plan=\"{plan}\";window.NEURON_CFG.pub_key=\"{pub_key}\";</script><script src=\"/js/checkout-stripe.js\" defer></script>" + free_init_script
-  max-width: 960px;
-  margin: 0 auto;
-  display: grid;
-  grid-template-columns: 1fr 1fr;
-  gap: 5rem;
-  align-items: start;
-}
-@media (max-width: 720px) {
-  .checkout-shell { grid-template-columns: 1fr; gap: 3rem; }
 }
+
+fn checkout_style_html() -> String {
+    <style>
 .checkout-plan-name {
   font-family: var(--head);
   font-size: clamp(1.5rem, 3vw, 2rem);
@@ -494,12 +488,4 @@ fn checkout_page(plan: String, pub_key: String) -> String {
 }
 .checkout-auth-badge strong { color: var(--navy); font-weight: 500; }
 </style>
-
-<script src="/js/checkout-auth.js" defer></script>
-
-<script>window.NEURON_CFG=window.NEURON_CFG||{};window.NEURON_CFG.plan="{plan}";window.NEURON_CFG.pub_key="{pub_key}";</script><script src="/js/checkout-stripe.js" defer></script>
-
-{#if is_free}
-<script>document.addEventListener('DOMContentLoaded',function(){window.neuronCheckoutFree&&window.neuronCheckoutFree()});</script>
-{/if}
 }

src/styles.el

@@ -1966,7 +1966,8 @@ fn page_open() -> String {
 }
 
 fn page_close() -> String {
-    return <script src="/js/chat-widget.js" defer></script>
+    let widgets: String = <div id="page-widgets">
+  <script src="/js/chat-widget.js" defer></script>
 
 <!-- ── Neuron Demo Chat Widget ──────────────────────────────────────────────── -->
 <div id="neuron-demo-btn">
@@ -2023,6 +2024,6 @@ fn page_close() -> String {
 </div>
 
 <script src="/js/styles.js" defer></script>
-</body>
+</div>
-</html>
+    return widgets + "</body></html>"
 }