feat: multi-document upload + streaming LLM responses

feat: Add multi-provider LLM support with UI model selector

docs: correct readme table

docs: overhaul READMEs for GitHub and HuggingFace

UI: ChatGPT-inspired dark theme - full-width, clean, usable

Minimal UI redesign + sales-focused READMEs with architecture diagrams

Enterprise demo transformation: UI redesign, sample docs, Docker, auto-cleanup