refactor: improve codebase structure and Univer integration

- Add AGENTS.md knowledge base with project documentation
- Move UserPreferences model to separate models.py file
- Extract API_KEY to environment variable for security
- Enhance Univer Editor with PPTX support and improved UI
- Improve file system handling with binary file detection
- Add HF_ENDPOINT mirror for better China connectivity
- Clean up unused imports and code structure

backend/tts_asr.py

@@ -1,9 +1,13 @@
 import asyncio
 import base64
 import logging
+import os
 from io import BytesIO
 from typing import Optional
 
+# 设置 Hugging Face 镜像源为国内镜像
+os.environ.setdefault("HF_ENDPOINT", "https://hf-mirror.com")
+
 import torch
 from fastapi import APIRouter, HTTPException
 from pydantic import BaseModel
@@ -29,8 +33,8 @@ def _get_device_map() -> str:
     try:
         if hasattr(torch.backends, "mps") and torch.backends.mps.is_available():
             return "mps"
-    except Exception:
-        pass
+    except Exception as e:
+        logger.debug("MPS check failed: %s", e)
     return "cpu"