infra: Add multi-stage frontend build to Docker

Nginx container now builds the Vue frontend in a Node stage
instead of mounting local dist/ files. This means:
- No need to commit dist/ or build locally before deploying
- docker compose up --build handles everything end-to-end
- Removed obsolete compose version key

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker/Dockerfile.nginx

@@ -0,0 +1,21 @@
+# Multi-stage build: compile Vue frontend, serve with nginx
+# Stage 1: Build the frontend
+FROM node:22-alpine AS builder
+
+WORKDIR /build
+
+# Cache dependencies — copy manifests first
+COPY package.json package-lock.json* ./
+RUN npm install
+
+# Copy source and build
+COPY . .
+RUN npm run build
+
+# Stage 2: Serve with nginx
+FROM nginx:alpine
+
+# Copy built frontend
+COPY --from=builder /build/dist /usr/share/nginx/html
+
+EXPOSE 80