sidebar: show full question text for conversations and saved responses

chat-ui: profession select default option → '-- please select --'

chat-ui: move new conversation button above saved responses in bottom block

chat-ui: group saved responses + past conversations at sidebar bottom

chat-ui: move download saved responses below saved responses section

chat-ui: pin past conversations to bottom of sidebar

Refactor: replace Streamlit with React+Vite SPA + FastAPI server