feat: add web server (FastAPI) and web UI on port 8001
- server.py: FastAPI + WebSocket server wrapping Brain, TTS, Actions
- WS /ws/chat: streaming chat with token-by-token delivery
- GET /api/audio/{filename}: serve generated TTS audio
- GET /api/health: server status check
- Serves static web UI from web/ directory
- web/: self-contained HTML/CSS/JS frontend
- Responsive chat interface with message bubbles
- WebSocket client for real-time streaming
- Voice input via Web Speech API (mic button)
- TTS audio playback (auto + manual replay)
- Conversation sidebar with history
- Settings panel (voice, TTS, sidebar toggles)
- Dark mode support via prefers-color-scheme
- Updated requirements.txt with fastapi, uvicorn, websockets
- Updated .env.example with SERVER_HOST/PORT config
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