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/pricing.el

@@ -53,6 +53,7 @@ fn pricing(sold: Int, total: Int) -> String {
           <li><span class=\"dash\">-</span><span>Persistent memory &#8212; never resets</span></li>
           <li><span class=\"dash\">-</span><span>Neuron inference &#8212; metered, priced below OpenAI and Anthropic</span></li>
           <li><span class=\"dash\">-</span><span>Local inference via Ollama</span></li>
+          <li style=\"list-style:none\"><span style=\"font-family:var(--body);font-size:0.8rem;font-weight:300;color:var(--t3);font-style:italic;line-height:1.6\">Local models may give lower quality responses. Neuron inference is cheap &#8212; give it a try.</span></li>
           <li><span class=\"dash\">-</span><span>Bring your own API keys (optional)</span></li>
           <li><span class=\"dash\">-</span><span>Unlimited projects</span></li>
           <li><span class=\"dash\">-</span><span>3 marketplace plugins included</span></li>