None of the response paths in `internal/web/` or `internal/api/` set the standard browser-security headers. `grep` for `Content-Security-Policy`, `X-Frame-Options`, `Strict-Transport-Security`, `X-Content-Type-Options`, `Referrer-Policy` returns zero matches across the codebase.

## Impact
The admin UI signs CA certificates, mints API keys (returned inline once per page), displays TOTP QR codes, and exposes operator-management forms. Missing `X-Frame-Options: DENY` / `frame-ancestors 'none'` is a real clickjacking lever against an admin browsing `/ui/operators/*` or `/ui/cas/*`. Missing `X-Content-Type-Options: nosniff` allows MIME confusion on any user-supplied content surface. Missing HSTS on TLS deployments leaves a downgrade window.

## Affected
All released versions up to v0.3.0.

## Suggested fix
A single response-header middleware mounted at the chi router root in both `/ui/*` and `/api/*` paths:

```go
func securityHeadersMiddleware(next http.Handler) http.Handler {
return http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) {
h := rw.Header()
h.Set("Content-Security-Policy",
"default-src 'self'; frame-ancestors 'none'; base-uri 'none'; form-action 'self'")
h.Set("X-Content-Type-Options", "nosniff")
h.Set("Referrer-Policy", "same-origin")
h.Set("X-Frame-Options", "DENY") // belt-and-braces; CSP frame-ancestors is the modern path
if r.TLS != nil {
h.Set("Strict-Transport-Security", "max-age=31536000; includeSubDomains")
}
next.ServeHTTP(rw, r)
})
}
```

The inline `<script>` in `layout.html` for CSRF wiring (added in the CSRF advisory) will need either a nonce, a hash in CSP, or external-file extraction. Easiest path: a nonce per request (`crypto/rand`, base64) injected into both the CSP header and the script's `nonce=""` attribute.