Add Dockerfile + nginx for Coolify deployment

Multi-stage build (node -> nginx) serving the Vite static output on
port 80, matching the dockerfile build pack used for other apps.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,13 @@
+# Build the Vite app, then serve the static output with nginx.
+FROM node:20-alpine AS build
+WORKDIR /app
+COPY package*.json ./
+RUN npm ci
+COPY . .
+RUN npm run build
+
+FROM nginx:alpine
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+COPY --from=build /app/dist /usr/share/nginx/html
+EXPOSE 80
+CMD ["nginx", "-g", "daemon off;"]