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.el

@@ -50,7 +50,7 @@ fn enterprise() -> String {
       <div class=\"enterprise-cap card-dark reveal\" style=\"transition-delay:390ms\">
         <h3 class=\"display-md\">Compliance architecture</h3>
         <div class=\"enterprise-cap-rule\"></div>
-        <p class=\"enterprise-cap-body\">SQLite local storage. No cloud database with your data. SOC 2 alignment built into the data model. ExternalSecret-based secrets management. Audit logs at every layer.</p>
+        <p class=\"enterprise-cap-body\">Custom on-device storage. No cloud database with your data. SOC 2 alignment built into the data model. ExternalSecret-based secrets management. Audit logs at every layer.</p>
       </div>
     </div>