Enhance `main.py`: add `multiprocessing.freeze_support()` for compatibility and improve `lib_path` existence check before modifying sys.path.

main.py

@@ -3,12 +3,13 @@ import sys
 import time
 import typing
 from pathlib import Path
+import multiprocessing
 
 import uvicorn
 
 HERE = Path(__file__).parent
 lib_path = HERE / "src"
-if lib_path.as_posix() not in sys.path:
+if lib_path.exists() and lib_path.as_posix() not in sys.path:
     sys.path.insert(0, lib_path.as_posix())
 
 from voice_dialogue.core.constants import (
@@ -346,4 +347,5 @@ VoiceDialogue - 语音对话系统
 
 
 if __name__ == '__main__':
+    multiprocessing.freeze_support()
     main()