nebula-mesh is a self-hosted control plane for Slack Nebula mesh virtual private network. Prior to version 0.3.1, 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. Version 0.3.1 fixes the issue.

Affected: Go: github.com/juev/nebula-mesh

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",…