fix(auth): set the session cookie's Secure flag behind a TLS-terminating proxy (M462 #4977)

The cookie set Secure from r.TLS, which is nil whenever TLS terminates at
Traefik or Cloudflare, so a public deployment's session cookie went out
without Secure. auth.IsHTTPS now decides it from X-Forwarded-Proto, believed
only through the trusted-hop count (rule 94) and read positionally like
X-Forwarded-For. Plain-HTTP and LAN logins are unchanged; nothing redirects.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/auth_register.go

@@ -195,7 +195,7 @@ func (h *handlers) handleRegister(w http.ResponseWriter, r *http.Request) {
 		Value:    sessionToken,
 		Path:     "/",
 		HttpOnly: true,
-		Secure:   r.TLS != nil,
+		Secure:   auth.IsHTTPS(r, h.netSettings.Hops()), // plain HTTP and LAN stay functional (rule 94)
 		SameSite: http.SameSiteStrictMode,
 		MaxAge:   int(sessionCookieMaxAge.Seconds()),
 	})