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

frontend/Dockerfile

@@ -1,6 +1,11 @@
 FROM oven/bun:1 AS build
 WORKDIR /app
 
+ARG VITE_WS_URL=""
+ARG VITE_WS_TOKEN=""
+ENV VITE_WS_URL=$VITE_WS_URL
+ENV VITE_WS_TOKEN=$VITE_WS_TOKEN
+
 COPY package.json bun.lock* ./
 RUN bun install --frozen-lockfile 2>/dev/null || bun install