fix: pre-download GGUF and embedding models at build time to eliminate cold-start delay

feat: enhance initialization status messages and add model download callback

update README.md

refactor: replace GCNConv with SAGEConv in VGAEEncoder and update forward method to return mu and log_std

feat: implement model path resolution for GGUF files in app.py and kg_builder.py

Add all required files for GraphRAG