Add static host header template

[TheSilkky]

Summary

• Add an adaptable _headers-style static-host security header template using placeholder origins only.
• Link the template from the browser security header docs and docs index.
• Document deployment-review boundaries, private admin route exclusions, cache guidance, and denied browser permissions.

Closes #54

Validation

• npm run typecheck
• npm run lint
• npm run test
• npm run build
• npm run test:e2e
• git diff --check