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/llms.txt

@@ -1,8 +1,8 @@
 # Hesychia
 
-> Hesychia is a calm AI assistant. Quiet, focused AI chat — private by default.
+> Hesychia is a calm AI assistant with quiet, focused AI chat that stays private by default.
 
-Hesychia is a $10/month AI chat product from AI ML Operations, LLC. It stores your documents so answers can use your files, and company managers can manage team accounts. Conversation memory is a setting you control. Canceling a subscription does not delete documents. Account deletion is a soft-delete.
+Hesychia is a $10/month AI chat product from AI ML Operations, LLC. It stores your documents so answers can use your files, and company managers can manage team accounts. Conversation memory is a setting you control, and canceling a subscription does not delete documents. Account deletion is a soft-delete, and stored account data may remain.
 
 ## Pages