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>

Main.cs

@@ -395,7 +395,6 @@ namespace KCM
                 // Your code here
 
                 // Get the name of the last method that called OnPlayerPlacement
-                string callTree = "";
                 List<string> strings = new List<string>();
 
                 for (int i = 1; i < 10; i++)

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

ServerLobby/ServerLobbyScript.cs

@@ -60,8 +60,6 @@ namespace KCM
             Falle
         }
 
-        bool awake = false;
-
         public void Start()
         {
             Main.helper.Log("ServerLobby start called");