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/llm.py

@@ -17,7 +17,6 @@ VLM_MODEL = os.getenv('VLM_MODEL', 'qwen3-vl:30b')
 # Timeouts in seconds (10 minutes for large model loading)
 COMPLETION_TIMEOUT = 600
 OCR_TIMEOUT = 600
-CONVERT_TIMEOUT = 600
 
 client = ollama.AsyncClient(host=OLLAMA_HOST)
 logger = logging.getLogger("llm")