fix(mqtt): add resilient ipv4 fallback and retry status UI

[dbortoluzzi]

This PR improves MQTT connection robustness on networks where broker.hivemq.com:1883 over IPv6 may fail.

What changed

• Added automatic fallback to DNS-resolved IPv4 addresses:
  • resolveIpv4Candidates in commonMain (expect)
  • Android implementation (InetAddress.getAllByName + Inet4Address filter)
  • iOS implementation (getaddrinfo + AF_INET filter)
• Updated MqttMailbox to try hosts in sequence:
  • DNS hostname
  • automatically resolved IPv4 candidates
• Kept periodic retry logic in the ViewModel without app crashes
• Added UI feedback message when connection fails and retry is in progress