refactor(standalone): 收敛 _STANDALONE 判断到 standalone_proxy 单一来源

各模块不再各自读 os.environ，统一从 standalone_proxy._STANDALONE 导入，
环境变量只在一处决定模式，减少散落的重复逻辑。

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

kilostar/api/__init__.py

@@ -20,7 +20,8 @@ from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import FileResponse, JSONResponse
 from fastapi.staticfiles import StaticFiles
 
-_STANDALONE = os.environ.get("KILOSTAR_MODE", "distributed") == "standalone"
+from kilostar.utils.standalone_proxy import _STANDALONE
+
 if not _STANDALONE:
     from ray import serve