Public homepage, pricing, privacy, changelog, and SEO (#117)

## Summary
- Logged-out `/` is a public landing page. Signed-in `/` still opens chat.
- Adds `/pricing/`, `/privacy/`, and `/changelog/`, plus header and footer links.
- Primary path is **Create account** when registration is on, and an email **waitlist** when it is off.
- Titles, canonicals, Organization and Product JSON-LD, `sitemap.xml`, `robots.txt`, and `llms.txt`.
- Usage counts render only from `GET /api/public/usage/`. No avatars. No founder block. No testimonials.
- Closes #113, #114, #115, and #116.

Needs [chat_backend#75](https://git.aimloperations.com/ai_ml_operations/chat_backend/pulls) for the waitlist and usage API, and the server-infra crawler-file change so those root files are not the SPA shell.

The account and document numbers in the screenshots come from a local mock of the usage API, not production.

## Screenshots

### Landing, registration off
![Landing waitlist desktop](https://git.aimloperations.com/ai_ml_operations/chat_web_app/raw/branch/feat/113-public-site/llm-fe/docs/ux/landing-desktop.png)

### Landing, mobile
![Landing mobile](https://git.aimloperations.com/ai_ml_operations/chat_web_app/raw/branch/feat/113-public-site/llm-fe/docs/ux/landing-mobile.png)

### Landing, registration on
![Landing create account](https://git.aimloperations.com/ai_ml_operations/chat_web_app/raw/branch/feat/113-public-site/llm-fe/docs/ux/landing-signup.png)

### Pricing
![Pricing](https://git.aimloperations.com/ai_ml_operations/chat_web_app/raw/branch/feat/113-public-site/llm-fe/docs/ux/pricing.png)

### Privacy
![Privacy](https://git.aimloperations.com/ai_ml_operations/chat_web_app/raw/branch/feat/113-public-site/llm-fe/docs/ux/privacy.png)

### Changelog
![Changelog](https://git.aimloperations.com/ai_ml_operations/chat_web_app/raw/branch/feat/113-public-site/llm-fe/docs/ux/changelog.png)

### Sign in unchanged
![Sign in](https://git.aimloperations.com/ai_ml_operations/chat_web_app/raw/branch/feat/113-public-site/llm-fe/docs/ux/signin.png)

## Test plan
- [x] `react-scripts test` — 234 passed
- [ ] Incognito `/` shows the landing page, not Sign In
- [ ] Signed-in `/` still opens chat
- [ ] `/account/` still redirects to `/signin/` when logged out
- [ ] Waitlist POST and usage GET against the new API
- [ ] `/robots.txt`, `/sitemap.xml`, and `/llms.txt` return plain text, not HTML

Reviewed-on: #117

llm-fe/public/index.html

@@ -8,11 +8,48 @@
   <link rel="icon" type="image/png" sizes="16x16" href="%PUBLIC_URL%/favicon-16x16.png" />
   <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover" />
   <meta name="theme-color" content="#000000" />
-  <meta name="description" content="Hesychia — a calm AI assistant by AI ML Operations, LLC" />
+  <meta name="description" content="Hesychia is a calm AI assistant. Quiet, focused AI chat — private by default. $10/month from AI ML Operations, LLC." />
   <meta name="application-name" content="Hesychia" />
-  <meta property="og:title" content="Hesychia" />
-  <meta property="og:description" content="Hesychia — quiet, focused AI chat at hesychia.ai" />
-  <meta property="og:url" content="https://hesychia.ai" />
+  <link rel="canonical" href="https://hesychia.ai/" />
+  <meta property="og:type" content="website" />
+  <meta property="og:title" content="Hesychia — quiet, focused AI chat" />
+  <meta property="og:description" content="Hesychia is a calm AI assistant. Quiet, focused AI chat — private by default. $10/month." />
+  <meta property="og:url" content="https://hesychia.ai/" />
+  <script type="application/ld+json">
+    {
+      "@context": "https://schema.org",
+      "@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>
+  <script type="application/ld+json">
+    {
+      "@context": "https://schema.org",
+      "@type": "SoftwareApplication",
+      "name": "Hesychia",
+      "applicationCategory": "BusinessApplication",
+      "operatingSystem": "Web",
+      "url": "https://hesychia.ai/",
+      "description": "A calm AI assistant. Quiet, focused AI chat — private by default.",
+      "offers": {
+        "@type": "Offer",
+        "price": "10.00",
+        "priceCurrency": "USD"
+      },
+      "provider": {
+        "@type": "Organization",
+        "name": "AI ML Operations, LLC",
+        "url": "https://www.aimloperations.com"
+      }
+    }
+  </script>
   <link rel="apple-touch-icon" sizes="180x180" href="%PUBLIC_URL%/apple-touch-icon.png" />
   <!--
       manifest.json provides metadata used when your web app is installed on a
@@ -28,7 +65,7 @@
       work correctly both with client-side routing and a non-root public URL.
       Learn how to configure a non-root public URL by running `npm run build`.
     -->
-  <title>Hesychia</title>
+  <title>Hesychia — quiet, focused AI chat</title>
 </head>
 
 <body>