Gentoo client install support

I'm running Gentoo with OpenRC too ( no systemd )

I can run the binary for the client, but I get an issue with

```
[2026-02-14][23:02:59.838879614][INFO][defguard_client] App setup completed, log level: INFO
[2026-02-14][23:02:59.838970923][INFO][defguard_client] Application data (database file) will be stored in: /home/user/.local/share/net.defguard and application logs in: /home/user/.local/share/net.defguard/logs. Logs of the background Defguard service responsible for managing VPN connections at the network level will be stored in: /var/log/defguard-service.

(defguard-client-linux-x86_64-v1.6.5:17714): libayatana-appindicator-WARNING **: 18:02:59.854: libayatana-appindicator is deprecated. Please use libayatana-appindicator-glib in newly written code.
[2026-02-14][23:02:59.858223529][ERROR][defguard_client] Failed to configure system tray: Tauri error: No such file or directory (os error 2)
[2026-02-14][23:02:59.908925851][INFO][defguard_client::enterprise::periodic::config] Automatically retrieved the newest instance configuration from core for 1 instances, sleeping for 30s
[2026-02-14][23:03:00.488839449][INFO][defguard_client::commands] The latest release version of the application available for download is 1.6.4, it was released on 2026-01-16.
[2026-02-14][23:03:00.821541371][INFO][defguard_client::commands] The latest release version of the application available for download is 1.6.4, it was released on 2026-01-16.
[2026-02-14][23:03:02.835066417][ERROR][defguard_client::service::client] Problem connecting to UNIX domain socket: No such file or directory (os error 2)
[2026-02-14][23:03:02.835119685][ERROR][defguard_client::utils] Failed to set up connection for location poc-vpn(ID: 1); background service is unavailable. Make sure the service is running. Error: code: 'The service is currently unavailable', message: "No such file or directory (os error 2)", source: tonic::transport::Error(Transport, ConnectError(Os { code: 2, kind: NotFound, message: "No such file or directory" })), Interface configuration: InterfaceConfiguration { name: "wg0", addresses: [IpAddrMask { address: 10.10.0.3, cidr: 32 }], port: 0, peers: [Peer { public_key: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxd, protocol_version: None, endpoint: Some(x.x.x.x:51820), last_handshake: None, tx_bytes: 0, rx_bytes: 0, persistent_keepalive_interval: Some(25), allowed_ips: [IpAddrMask { address: 10.100.0.0, cidr: 16 }], .. }], mtu: None, fwmark: None, .. }
[2026-02-14][23:03:02.836748435][ERROR][webview] Invoking "connect" failed due to unknown error: "Internal error: Background service is unavailable. Make sure the service is running."
[2026-02-14][23:03:02.837297159][ERROR][webview] Error handling interface: Invoking "connect" failed due to unknown error: "Internal error: Background service is unavailable. Make sure the service is running."
```

Reproduziere den Client-Start auf Gentoo mit OpenRC und überprüfe den gemeldeten system-tray-Fehler sowie den fehlenden UNIX-domain socket in den Logs. Ermittle die beteiligten Einstiegspunkte für die Installation und den Hintergrunddienst; abgeschlossen ist die Aufgabe, wenn der Client starten kann, der Dienst verfügbar ist und der gemeldete Verbindungsfehler ohne systemd nicht mehr auftritt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.