build: 添加 Flutter Web 构建产物 + 修复部署脚本

- 本地构建 Flutter Web (flutter build web --release)
- 服务器无需 Flutter 环境，直接复制 build/web
- 修复 403 错误（build/web 目录为空导致）

deploy/deploy_server.sh

@@ -35,16 +35,9 @@ log() {
 sync_frontend() {
     log "📂 [前端] 开始构建..."
 
-    # Flutter Web 构建
-    # log "4.1 构建 Flutter Web..."
-    # cd $PROJECT_DIR/flutter_monisuo
-    # flutter build web --release --dart-define=ENV=prod >> $BUILD_LOG 2>&1
-    # if [ $? -eq 0 ]; then
-    #     log "✅ Flutter Web 构建完成"
-    # else
-    #     log "❌ Flutter Web 构建失败"
-    #     exit 1
-    # fi
+    # Flutter Web - 从 Git 拉取的 build/web 复制（服务器无 Flutter 环境）
+    # 本地需先构建：flutter build web --release --dart-define=ENV=prod
+    # 然后 git add flutter_monisuo/build/web/ && git commit && git push
 
     # 同步 Flutter Web
     FLUTTER_BUILD_DIR="$PROJECT_DIR/flutter_monisuo/build/web"