feat: add production deployment config

- Add docker-compose.prod.yml with env var support
- Add frontend/Dockerfile.prod with nginx for static serving
- Fix Zulip notification to run in thread pool (avoid blocking)
- Use Zulip time format for timezone-aware display
- Add Zulip @mentions for users matched by email

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/Dockerfile.prod

@@ -0,0 +1,34 @@
+# Build stage
+FROM node:20-alpine AS builder
+
+WORKDIR /app
+
+COPY package*.json ./
+RUN npm ci
+
+COPY . .
+
+# VITE_API_URL must be set at build time
+ARG VITE_API_URL
+ENV VITE_API_URL=${VITE_API_URL}
+
+RUN npm run build
+
+# Production stage
+FROM nginx:alpine
+
+COPY --from=builder /app/dist /usr/share/nginx/html
+
+# Handle client-side routing
+RUN echo 'server { \
+    listen 8080; \
+    root /usr/share/nginx/html; \
+    index index.html; \
+    location / { \
+        try_files $uri $uri/ /index.html; \
+    } \
+}' > /etc/nginx/conf.d/default.conf
+
+EXPOSE 8080
+
+CMD ["nginx", "-g", "daemon off;"]