Fix initStripe load order, subscription webhook email extraction, chat textarea UX

- checkout.el: swap stripe_el_script before auth_script so initStripe is
  defined when Supabase auth fires onAuthStateChange on page load
- main.el: fix Stripe webhook email extraction for checkout.session.completed
  (subscription) events — customer_details is nested at data.object level,
  not at root; previous code only worked for payment_intent.succeeded
- page_close.c: replace <input type="text"> with <textarea rows="1"> in
  the chat widget input row so long questions are visible as you type
- page_css.c: update #neuron-demo-text CSS for textarea (resize:none,
  overflow:hidden, min/max-height, align-items:flex-end on row)
- chat-widget.el: add auto-resize event listener (grows up to ~4 lines),
  reset height to auto on send

src/js/chat-widget.el

@@ -497,6 +497,7 @@ fn main() -> Void {
       return;
     }
     input.value = '';
+    input.style.height = 'auto';
     btn.disabled = true;
     addMsg('user', msg);
 
@@ -617,6 +618,10 @@ fn main() -> Void {
     inp.addEventListener('keydown', function(e) {
       if (e.key === 'Enter' && !e.shiftKey) { e.preventDefault(); window.neuronDemoSend(); }
     });
+    inp.addEventListener('input', function() {
+      this.style.height = 'auto';
+      this.style.height = Math.min(this.scrollHeight, 120) + 'px';
+    });
   }
 })()")
 }