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/models.py

@@ -0,0 +1,9 @@
+"""共享的 Pydantic 模型定义"""
+from pydantic import BaseModel
+
+
+class UserPreferences(BaseModel):
+    """用户偏好设置"""
+    language: str = "auto"
+    currency: str = "auto"
+    timezone: str = "auto"