Dark theme and session delete ...

- Switch to dark color palette via CSS custom properties
- Delete button appears on hover for each session item
- Deleting active session resets chat to empty state

Add web chat client ...

- Single-page chat UI served as static files from FastAPI
- Sidebar with profile selector, new chat button, session list
- WebSocket streaming: shows tool calls and text deltas in real time
- Typing indicator, streaming cursor, auto-resize textarea
- Session history loads on switch