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/prompt.py

@@ -1,17 +1,11 @@
 from datetime import datetime, timedelta, timezone
 import re
-from typing import Protocol, Tuple, runtime_checkable
+from typing import Tuple
 
+from models import UserPreferences
 from prompts import get_language_guidance_map, get_system_prompt_template, get_inline_examples
 
 
-@runtime_checkable
-class UserPreferences(Protocol):
-    language: str
-    currency: str
-    timezone: str
-
-
 def _get_current_datetime(timezone_pref: str = "auto") -> str:
     # Default to UTC+8 if auto or not specified.
     offset = 8