Restyle sidebar with Neon card layout, refine buttons and labels

Simplify model selection: single-click to reuse, replace oldest on overflow

Rebrand UI with Neon.ai header, sidebar styling, expert personas, and UX improvements

Initial commit: LLMChats3 - LLM conversation app with React + FastAPI, persona config, orchestrated conversation lifecycle, collapsible sidebar accordions, stop button, and export capabilities