nebula-mesh: Session and OIDC state cookies lack the Secure attribute

internal/web/session.go and internal/web/oidc.go set HttpOnly and SameSite=Lax on every cookie but never Secure. A single plaintext request to the origin (operator on a LAN, mistyped URL, HTTP→HTTPS not strictly enforced, reverse proxy misconfiguration) discloses the session.

Affected

All released versions up to v0.3.1.

Impact

An attacker who can observe one HTTP request to the origin recovers the session cookie and impersonates the operator for the remainder of its 24h TTL. The OIDC state cookie has a narrower 10-minute window but enables CSRF on the OIDC callback during that window.

## Cookie sites - internal/web/session.go — Login, StartAuthenticatedSession, CompleteTwoFactor, Logout - internal/web/oidc.go — HandleLogin (state set), HandleCallback (state clear)

Suggested fix

Driven by an explicit cookie_secure config option, inferred true when tls_cert+tls_key are configured and false otherwise. rate_limit.trust_proxy_header is deliberately not used as a signal — that flag controls XFF parsing for rate-limit IPs and does not promise the proxy speaks TLS to clients. Operator behind a TLS-terminating proxy sets cookie_secure: true explicitly.

Logout and OIDC state-clear cookies also pick up matching HttpOnly + SameSite=Lax so browsers reliably replace the original.

Reproducer

Start nebula-mgmt without tls_cert/tls_key (the documented "behind a reverse proxy" deployment). Hit any login flow over the local listener:

curl -i -X POST -d 'username=admin&password=…' http://127.0.0.1:8080/ui/login

The Set-Cookie: nebula_session=… line will lack Secure. A subsequent unencrypted hop reveals the cookie verbatim.

Operational migration

Operators flipping cookie_secure on a running deployment should expect a one-time logout: existing browser cookies have the old attribute set and the new delete-cookie won't match.