Public page SEO and production analytics (#125)

## Summary
- Closes the public-page gaps from #124: each route gets its own title, description, canonical, heading, links, and JSON-LD, and unknown URLs have a real 404 document.
- Adds HeyCatch and the Google tag (`G-V12DLRLPWR`) only on the production site and the iOS and Android apps. Tianji still loads on beta.
- Needs the server-infra change that stops serving the app shell for missing paths.

## Test plan
- [ ] Build production and confirm `/` and `/pricing/` HTML include a unique title, description, canonical, one h1, social tags, related links, and Organization JSON-LD.
- [ ] Confirm a nonsense path is served as `404.html` once the nginx change is deployed.
- [ ] Confirm beta and local web do not load HeyCatch or gtag, and that a production or native build does.
- [ ] Open the live site after deploy and click around, then mark HeyCatch installed in its dashboard.

Reviewed-on: #125

llm-fe/public/404.html

@@ -0,0 +1,48 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+  <meta charset="utf-8" />
+  <title>The page is not found — Hesychia</title>
+  <meta name="description" content="The page is not found. This address is not a Hesychia page. Open the home page to read what the product does, what it costs, and how to start an account." />
+  <meta property="og:title" content="The page is not found — Hesychia" />
+  <meta property="og:description" content="The page is not found. This address is not a Hesychia page. Open the home page to read what the product does, what it costs, and how to start an account." />
+  <meta property="og:url" content="https://hesychia.ai/404.html" />
+  <meta property="og:image" content="https://hesychia.ai/og-image.png" />
+  <meta property="og:image:width" content="1200" />
+  <meta property="og:image:height" content="630" />
+  <meta name="twitter:card" content="summary_large_image" />
+  <meta name="twitter:title" content="The page is not found — Hesychia" />
+  <meta name="twitter:description" content="The page is not found. This address is not a Hesychia page. Open the home page to read what the product does, what it costs, and how to start an account." />
+  <meta name="twitter:image" content="https://hesychia.ai/og-image.png" />
+  <script type="application/ld+json">
+{
+  "@context": "https://schema.org",
+  "@graph": [
+    {
+      "@type": "WebPage",
+      "headline": "The page is not found — Hesychia",
+      "url": "https://hesychia.ai/404.html",
+      "mainEntityOfPage": "https://hesychia.ai/404.html"
+    },
+    {
+      "@type": "Organization",
+      "name": "Hesychia",
+      "url": "https://hesychia.ai",
+      "logo": "https://hesychia.ai/apple-touch-icon.png",
+      "parentOrganization": {
+        "@type": "Organization",
+        "name": "AI ML Operations, LLC",
+        "url": "https://www.aimloperations.com"
+      }
+    }
+  ]
+}
+</script>
+</head>
+<body>
+  <h1>Not Found</h1>
+  <p>The page is not found, and this address is not a Hesychia page.</p>
+  <p>Open the home page to read what the product does, what it costs, and how to start an account.</p>
+  <nav aria-label="Related pages"><a href="/">What Hesychia is</a><a href="/pricing/">Hesychia pricing — $10/month, no ads</a><a href="/privacy/">What Hesychia stores and does not sell</a><a href="/changelog/">Dated Hesychia product updates</a></nav>
+</body>
+</html>