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.js

@@ -3523,7 +3523,7 @@ var EnhancedSecureP2PChat = () => {
       } catch (error) {
       }
     }
-    handleMessage(" SecureBit.chat Enhanced Security Edition v5.4.5 - ECDH + DTLS + SAS initialized. Ready to establish a secure connection with ECDH key exchange, DTLS fingerprint verification, and SAS authentication to prevent MITM attacks.", "system");
+    handleMessage(" SecureBit.chat Enhanced Security Edition v5.4.10 - ECDH + DTLS + SAS initialized. Ready to establish a secure connection with ECDH key exchange, DTLS fingerprint verification, and SAS authentication to prevent MITM attacks.", "system");
     manager.setFileTransferCallbacks(
       // Progress callback — drives the voice-note upload/download ring.
       (progress) => {