Add integrated checkout, child safety section, and content polish

- checkout.el: Custom Stripe Payment Element page at /checkout?plan=...
- safety.el: Hard Bell and emergency routing architecture section
- main.el: Wire checkout + safety, /api/payment-intent, STRIPE_PUBLISHABLE_KEY
- enterprise_terms.el: Add nav bar (was missing, page had no navigation)
- styles.el: Checkout buttons now link to integrated page, not hosted Stripe
- El runtime: Auto-detect HTML responses, serve text/html vs application/json
- footer.el: Wordmark image centered above "Built Different." tagline
- environmental.el: Replace SQLite with custom on-device storage
- enterprise.el: Replace SQLite with custom on-device storage
- pricing.el: Local models degraded performance note near Ollama feature
- about.el: "left home at fifteen", remove programming language mention

src/footer.el

@@ -6,8 +6,8 @@ fn footer() -> String {
   <div class=\"container\">
     <div class=\"footer-inner\">
 
-      <a href=\"/\" class=\"footer-brand\" aria-label=\"Neuron home\">
-        <p class=\"footer-brand-name\">Neuron</p>
+      <a href=\"/\" class=\"footer-brand\" aria-label=\"Neuron home\" style=\"display:flex;flex-direction:column;align-items:center;\">
+        <img src=\"/assets/brand/neuron-wordmark-on-light.png\" srcset=\"/assets/brand/neuron-wordmark-on-light@2x.png 2x\" alt=\"Neuron\" height=\"24\" style=\"display:block;margin-bottom:0.35rem;\">
         <p class=\"footer-brand-tagline\">Built Different.</p>
       </a>