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/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>
-</html>
+</div>
+    return widgets + "</body></html>"
 }