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

Keep height:100vh scroll containment at all breakpoints to fix HF iframe

Add responsive design for phone and tablet screens

Fix sidebar model name overflow with ellipsis truncation and flex-shrink protection

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