Refactor VPN configuration handling to remove device limit management

- Updated the VPN configuration commands and handlers to eliminate the device limit parameter, simplifying the configuration process.
- Adjusted related API documentation to reflect the removal of device limit management, clarifying that this setting is now handled directly in the 3x-ui by node administrators.
- Enhanced the overall codebase by removing unnecessary device limit references across various components, ensuring a cleaner and more maintainable code structure.

backend/src/PnvPanel.Application/Configs/Rotate/RotateVpnConfigCommandHandler.cs

@@ -35,7 +35,7 @@ public sealed class RotateVpnConfigCommandHandler(IAppDbContext dbContext, IXuiP
         var newClientEmail = VpnConfig.GenerateClientEmail(userId);
         var addResult = await gateway.AddClientAsync(
             node, inbound.RemoteInboundId, config.Protocol, newClientEmail,
-            config.Label ?? newClientEmail, config.DeviceLimit, cancellationToken);
+            config.Label ?? newClientEmail, cancellationToken);
 
         if (!addResult.IsSuccess)
             return Result.Failure<VpnConfigDto>(addResult.Error);