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/sitemap.xml

@@ -0,0 +1,27 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+  <url>
+    <loc>https://hesychia.ai/</loc>
+    <lastmod>2026-10-17</lastmod>
+  </url>
+  <url>
+    <loc>https://hesychia.ai/pricing/</loc>
+    <lastmod>2026-10-17</lastmod>
+  </url>
+  <url>
+    <loc>https://hesychia.ai/privacy/</loc>
+    <lastmod>2026-10-17</lastmod>
+  </url>
+  <url>
+    <loc>https://hesychia.ai/changelog/</loc>
+    <lastmod>2026-10-17</lastmod>
+  </url>
+  <url>
+    <loc>https://hesychia.ai/terms_of_service/</loc>
+    <lastmod>2026-07-27</lastmod>
+  </url>
+  <url>
+    <loc>https://hesychia.ai/account-deletion/</loc>
+    <lastmod>2026-08-04</lastmod>
+  </url>
+</urlset>