fix(ios): use effective agent label for live activity

When the gateway has a default agent but the user has no explicit selection, the Dynamic Island should show the resolved active agent name instead of falling back to a hardcoded main placeholder.\n\nUse NodeAppModel.activeAgentName when starting the Live Activity so the label stays aligned with the effective gateway agent identity.

apps/ios/Sources/Model/NodeAppModel.swift

@@ -1955,7 +1955,7 @@ private extension NodeAppModel {
                         liveActivity.handleConnecting()
                     } else {
                         liveActivity.startActivity(
-                            agentName: self.selectedAgentId ?? "main",
+                            agentName: self.activeAgentName,
                             sessionKey: self.mainSessionKey)
                     }
                 }