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

feat(ui): overhaul UI with enterprise glassmorphism and matte theme

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

OpenRouter Added, Rate Limitng Fixed

Clean snapshot for Hugging Face Space (no large files)