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/enterprise_terms.el

@@ -9,6 +9,19 @@ fn enterprise_terms_page() -> String {
 
 fn enterprise_terms_body() -> String {
     return "
+<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=\"/#how-it-works\" class=\"nav-link\">How it works</a>
+      <a href=\"/#pricing\"       class=\"nav-link\">Pricing</a>
+      <a href=\"/#enterprise\"    class=\"nav-link\">Enterprise</a>
+      <a href=\"/about\"          class=\"nav-link\">About</a>
+      <a href=\"/#pricing\"       class=\"nav-cta\">Get Access</a>
+    </div>
+  </div>
+</nav>
+
 <div style=\"max-width:720px;margin:0 auto;padding:6rem 2.5rem 8rem\">
   <div style=\"margin-bottom:3rem\">
     <a href=\"/\" style=\"font-family:var(--body);font-size:0.75rem;font-weight:500;letter-spacing:0.15em;text-transform:uppercase;color:var(--navy);text-decoration:none\">&#8592; Neuron</a>