feat: tunnel host and port in the panel settings, ufw kept in sync

The direct tunnel port (8446 by default) and the host agents dial for
it can be set in Settings. The server moves its listeners at once and
agents pick the address up from the panel: on connect and with every
poll, unless TUNNEL_ADDR / TUNNEL_PORT pin it locally.

ufw: run with a C locale (the output is translated), do not adopt rules
that existed before the panel, skip loopback-only listeners, re-open
what saved forwards need on start and when the integration is turned
on, move the shared tunnel rule with the port and drop it when no
direct forward is left. The last ufw error is shown in Settings.

The forwards page shows the real tunnel and panel ports.

internal/server/handlers_agent.go

@@ -52,7 +52,31 @@ func (s *Server) handlePoll(w http.ResponseWriter, r *http.Request) {
 	} else {
 		_ = s.db.TouchAgent(a.ID)
 	}
-	writeJSON(w, s.hub.WaitControl(a.ID, pollHold))
+	msg := s.hub.WaitControl(a.ID, pollHold)
+	// Set after the hold so a port changed meanwhile reaches the agent now.
+	s.setTunnelHeader(w)
+	writeJSON(w, msg)
+}
+
+// tunnelAddrHeader carries the direct-tunnel address ("host:port", host empty
+// = the panel's host) to agents, on every poll and on /api/agent/config.
+const tunnelAddrHeader = "X-Tunnel-Addr"
+
+func (s *Server) setTunnelHeader(w http.ResponseWriter) {
+	if addr := s.tunnelAdvert(); addr != "" {
+		w.Header().Set(tunnelAddrHeader, addr)
+	}
+}
+
+// GET /api/agent/config — settings an agent picks up when it connects, without
+// waiting out a poll hold.
+func (s *Server) handleAgentConfig(w http.ResponseWriter, r *http.Request) {
+	if s.authenticateAgent(r) == nil {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+		return
+	}
+	s.setTunnelHeader(w)
+	w.WriteHeader(http.StatusNoContent)
 }
 
 // GET /api/agent/task/{id} — the agent fetches the task to run.