Fix compilation error and warnings

- LobbyManager.cs: Add missing 'using System;' for Exception class
- Main.cs: Remove unused 'callTree' variable
- ServerLobbyScript.cs: Remove unused 'awake' field

Fixes:
- Error: Exception type not found in LobbyManager.cs:177
- Warning: callTree variable assigned but never used
- Warning: awake field assigned but never used

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

RiptideSteamTransport/LobbyManager.cs

@@ -2,6 +2,7 @@
 using KCM.Enums;
 using KCM.Packets.Handlers;
 using Steamworks;
+using System;
 using UnityEngine;
 
 namespace Riptide.Demos.Steam.PlayerHosted