feat: single Docker image with frontend served from backend

- Dockerfile: 3-stage build (frontend npm build → backend go build → alpine runtime)
- Frontend dist files served from disk at /app/frontend/dist
- server.go: catch-all GET handler serves static files with SPA fallback to index.html
- config.go: new server.frontend_dir option (env: SUB_STORE_SERVER_FRONTEND_DIR)
- API routes (/api/*), download routes, and /health take priority over static serving

Dockerfile

@@ -1,4 +1,12 @@
-# ─── Stage 1: Build backend ───
+# ─── Stage 1: Build frontend ───
+FROM node:22-alpine AS frontend-builder
+WORKDIR /frontend
+COPY frontend/package.json frontend/package-lock.json ./
+RUN npm ci --production=false
+COPY frontend/ .
+RUN npm run build
+
+# ─── Stage 2: Build backend ───
 FROM golang:1.26-alpine AS backend-builder
 RUN apk add --no-cache git ca-certificates
 WORKDIR /app
@@ -7,17 +15,19 @@ RUN go mod download
 COPY . .
 RUN CGO_ENABLED=0 GOOS=linux go build -ldflags="-s -w" -o /sub-store .
 
-# ─── Stage 2: Runtime ───
+# ─── Stage 3: Runtime ───
 FROM alpine:3.21
 RUN apk add --no-cache ca-certificates tzdata
 WORKDIR /app
 COPY --from=backend-builder /sub-store /app/sub-store
+COPY --from=frontend-builder /frontend/dist /app/frontend/dist
 COPY config/config.example.yaml /app/config/config.example.yaml
 
 RUN mkdir -p /app/data
 
 ENV TZ=Asia/Shanghai
 ENV SUB_STORE_CONFIG=/app/config/config.yaml
+ENV SUB_STORE_FRONTEND_DIR=/app/frontend/dist
 
 EXPOSE 3000