feat(network): integrate nmrs for VPN, airplane mode, and secret agent

Replace cosmic-settings-network-manager-subscription channel-based
NetworkManager calls with direct nmrs API calls:
- VPN list: nm.list_saved_connections() instead of hand-walking
  NetworkManagerSettings over D-Bus.
- VPN connect/disconnect: nm.connect_vpn_by_uuid / disconnect_vpn_by_uuid.
- Forget Wi-Fi: nm.forget(&ssid).
- Wi-Fi radio toggle: nm.set_wireless_enabled.
- Airplane mode: nm.set_airplane_mode, dropping
  cosmic-settings-airplane-mode-subscription.
- Secret agent: nmrs::agent::SecretAgent registered per-popup,
  replacing nm-secret-agent-manager. VPN secrets are replied via
  responder.vpn_secrets(...); the applets own Authenticate flow still
  drives Wi-Fi password handoff and releases NM with NoSecrets.

cosmic-applet-network/Cargo.toml

@@ -26,16 +26,13 @@ tracing-log.workspace = true
 tracing-subscriber.workspace = true
 tracing.workspace = true
 zbus.workspace = true
-nm-secret-agent-manager = { git = "https://github.com/pop-os/dbus-settings-bindings/" }
 indexmap = "2.13.0"
 secure-string = "0.3.0"
 uuid = { version = "1.21.0", features = ["v4"] }
+nmrs = "3.0"
+
 
 
 [dependencies.cosmic-settings-network-manager-subscription]
 git = "https://github.com/pop-os/cosmic-settings/"
 # path = "../../cosmic-settings/subscriptions/network-manager"
-
-[dependencies.cosmic-settings-airplane-mode-subscription]
-git = "https://github.com/pop-os/cosmic-settings/"
-# path = "../../cosmic-settings/subscriptions/airplane-mode"