fix(mining-admin-web): ensure public dir exists in Docker build

mkdir -p public before build so COPY --from=builder /app/public works
even without static assets.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/mining-admin-web/Dockerfile

@@ -25,6 +25,9 @@ COPY . .
 ENV NEXT_TELEMETRY_DISABLED=1
 ENV NODE_ENV=production
 
+# 确保 public 目录存在（可能为空）
+RUN mkdir -p public
+
 # 构建应用
 RUN npm run build