Replace free TURN with self-hosted coturn; fix Safari connectivity; release v5.4.10

- config/ice-servers.prod.js: swap ExpressTURN for self-hosted coturn at
  turn.securebit.chat (TURN udp/tcp on 3478, TURNS/TLS on 443). Long-lived
  REST-API credential (expiry capped at int32 max for coturn compatibility).
- Add raw-IP STUN/TURN fallback (144.172.96.126): Safari's WebRTC layer fails
  to resolve STUN/TURN hostnames on some networks and gathers no srflx/relay
  candidates; reaching the server by IP fixes cross-browser (Safari<->Chrome)
  connections. Harmless to other browsers.
- deploy/nginx.conf: never long-cache /config/ice-servers.js so clients don't
  lock onto a stale server list.
- Bump version to 5.4.10 (header + init banner).

dist/app-boot.js

@@ -17721,7 +17721,7 @@ Right-click or Ctrl+click to disconnect`,
           React.createElement("div", { key: "txt", style: { lineHeight: 1.2, minWidth: 0 } }, [
             React.createElement("div", { key: "r1", style: { display: "flex", alignItems: "baseline", gap: "7px" } }, [
               React.createElement("span", { key: "n", style: { fontSize: "16px", fontWeight: 800, letterSpacing: "-0.3px", color: "#e8e8eb" } }, "SecureBit"),
-              React.createElement("span", { key: "v", style: { fontFamily: MONO, fontSize: "10px", fontWeight: 500, color: "#56565e" } }, "v5.4.5")
+              React.createElement("span", { key: "v", style: { fontFamily: MONO, fontSize: "10px", fontWeight: 500, color: "#56565e" } }, "v5.4.10")
             ]),
             React.createElement("div", { key: "r2", className: "hidden sm:block", style: { fontSize: "11px", color: "#6b6b73", fontWeight: 500 } }, "End-to-end encrypted")
           ])