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/main.py

@@ -17,6 +17,7 @@ from pydantic import BaseModel
 
 from geoip import get_ip_location_text
 from llm import call_ollama, call_vlm_ocr
+from models import UserPreferences
 from prompt import build_completion_prompts, prepare_prompt_context
 import markitdown
 
@@ -57,7 +58,7 @@ app.add_middleware(
     allow_headers=["*", "X-API-Key", "X-Client-IP", "X-Request-Id"],
 )
 
-API_KEY = "your-secret-key-here"
+API_KEY = os.getenv("API_KEY", "your-secret-key-here")
 api_key_header = APIKeyHeader(name="X-API-Key")
 
 
@@ -70,12 +71,6 @@ async def get_api_key(api_key: str = Security(api_key_header)):  # pragma: no co
     return api_key
 
 
-class UserPreferences(BaseModel):
-    language: str = "auto"
-    currency: str = "auto"
-    timezone: str = "auto"
-
-
 class CompletionRequest(BaseModel):
     prefix: str
     suffix: str