chore(ci): add heartbeat logs during flutter apk build

Print a timestamp every 30 seconds while flutter build runs so long
Gradle/download phases are visible in Jenkins instead of appearing hung.

Made-with: Cursor

Jenkinsfile

@@ -158,7 +158,14 @@ pipeline {
                             EXTRA_ARGS="\$EXTRA_ARGS --dart-define=APP_ENV=\$BUILD_ENV"
                             
                             echo "  - 开始构建 APK 参数: \$EXTRA_ARGS"
-                            fvm flutter build apk --release \$EXTRA_ARGS
+                            echo "  - 开始执行 flutter build apk（构建期间每30秒输出一次心跳）..."
+                            (fvm flutter build apk --release \$EXTRA_ARGS) &
+                            BUILD_PID=\$!
+                            while kill -0 \$BUILD_PID 2>/dev/null; do
+                                echo "  - 构建仍在进行中... \$(date '+%H:%M:%S')"
+                                sleep 30
+                            done
+                            wait \$BUILD_PID
                         """
                         echo "  ✅ ${currentApp} Android APK 构建完成"
                     }