feat: chat interface with gateway WebSocket integration (HQ-21)

- GatewayClient class: WS connection, auto-reconnect, request/response, events
- ChatPage with thread list sidebar + message area
- Real-time streaming responses via chat events
- Thread management with localStorage persistence
- Message bubbles with user/assistant/system styles
- Build args for VITE_WS_URL and VITE_WS_TOKEN
- SPA routing already supported by nginx config

docker-compose.dokploy.yml

@@ -34,6 +34,9 @@ services:
     build:
       context: ./frontend
       dockerfile: Dockerfile
+      args:
+        VITE_WS_URL: ${VITE_WS_URL:-wss://ws.hammer.donovankelly.xyz}
+        VITE_WS_TOKEN: ${VITE_WS_TOKEN}
     ports:
       - "80"
     depends_on: