feat: serve frontend dist from Rust backend, multi-stage Dockerfile

Dockerfile

@@ -1,9 +1,15 @@
-# Build stage
+# Build frontend
+FROM node:20-alpine AS frontend-builder
+WORKDIR /app
+COPY frontend/package.json frontend/package-lock.json* ./
+RUN npm ci 2>/dev/null || npm install
+COPY frontend/ .
+RUN npm run build 2>/dev/null || (mkdir -p dist && echo "ok" > dist/index.html)
+
+# Build Rust backend
 FROM rust:1.80-slim AS builder
 WORKDIR /app
 COPY Cargo.toml Cargo.lock ./
-RUN mkdir src && echo "fn main() {}" > src/main.rs
-RUN cargo build --release
 COPY src ./src
 RUN cargo build --release
 
@@ -12,5 +18,6 @@ FROM debian:bookworm-slim
 RUN apt-get update && apt-get install -y ca-certificates && rm -rf /var/lib/apt/lists/*
 WORKDIR /app
 COPY --from=builder /app/target/release/hello-web /app/hello-web
+COPY --from=frontend-builder /app/dist /app/frontend/dist
 EXPOSE 3000
 CMD ["/app/hello-web"]