feat(frontend): add multi-stage Dockerfile for Coolify static build

frontend/Dockerfile

@@ -0,0 +1,13 @@
+# --- Build stage ---
+FROM node:20-alpine AS build
+WORKDIR /app
+COPY package*.json ./
+RUN npm ci
+COPY . .
+RUN npm run build
+
+# --- Serve stage ---
+FROM nginx:alpine
+WORKDIR /usr/share/nginx/html/
+COPY --from=build /app/dist .
+COPY ./nginx.conf /etc/nginx/conf.d/default.conf