fix(wallpapercache): use QImageReader scaled decoding to avoid OOM on large images

1. Replace QImage::load() with QImageReader in pixmix effect processing,
   cap decoded resolution at 8K (7680px) to prevent excessive memory usage
2. Use QImageReader::setScaledSize() for scaleImage thread so decoding
   outputs directly at target size instead of full-size then downscale
3. Use QImageReader::imageFormat() static method for format detection to
   avoid instantiating a full reader when only the format is needed
4. Add proper error handling with canRead/isEmpty checks and error strings

Log: Optimize wallpaper image decoding to use scaled reading and prevent OOM on large images

fix(wallpapercache): 使用 QImageReader 缩放解码避免大图内存溢出

1. 将 pixmix 特效处理中的 QImage::load() 替换为 QImageReader，
   限制解码分辨率上限为 8K (7680px)，防止内存占用过大
2. 缩放线程使用 QImageReader::setScaledSize() 在解码时直接输出目标尺寸，
   避免先全尺寸加载再缩放带来的额外内存开销
3. 格式检测改用 QImageReader::imageFormat() 静态方法，
   避免实例化完整 reader 造成不必要的图片加载
4. 增加完善的错误检查，包含 canRead/isEmpty 校验及错误信息输出

Log: 优化壁纸图片解码流程，采用缩放读取方式防止超大图片导致内存溢出
PMS: BUG-359341

Author: mhduiy

src/plugin-qt/wallpapercache/imageeffectprocessor.cpp

@@ -17,6 +17,7 @@
 #define PIXMIX_OPACITY      90          // Color opacity
 #define PIXMIX_SATURATION   50          // Saturation
 #define PIXMIX_BRIGHTNESS   -60         // Brightness
+#define PIXMIX_MAX_DIM      7680        // 8K UHD long side cap
 
 DGUI_USE_NAMESPACE
 
@@ -67,9 +68,25 @@ ImageEffectProcessor::EffectType ImageEffectProcessor::effectTypeFromString(cons
 
 QImage ImageEffectProcessor::processPixmixEffect(const QString &imagePath)
 {
-    QImage originalImage;
-    if (!originalImage.load(imagePath)) {
-        qWarning() << "Failed to load image:" << imagePath;
+    QImageReader reader(imagePath);
+    if (!reader.canRead()) {
+        qWarning() << "Cannot read image:" << imagePath;
+        return QImage();
+    }
+
+    QSize originalSize = reader.size();
+    if (!originalSize.isEmpty()
+        && (originalSize.width() > PIXMIX_MAX_DIM || originalSize.height() > PIXMIX_MAX_DIM)) {
+        QSize decodeSize = originalSize;
+        decodeSize.scale(PIXMIX_MAX_DIM, PIXMIX_MAX_DIM, Qt::KeepAspectRatio);
+        reader.setScaledSize(decodeSize);
+        qDebug() << "Decoding image at" << decodeSize
+                 << "(original:" << originalSize << ")";
+    }
+
+    QImage originalImage = reader.read();
+    if (originalImage.isNull()) {
+        qWarning() << "Failed to load image:" << imagePath << reader.errorString();
         return QImage();
     }
 

src/plugin-qt/wallpapercache/scaleimagethread.cpp

@@ -10,6 +10,7 @@
 #include <QDateTime>
 #include <QCoreApplication>
 #include <QCryptographicHash>
+#include <QImageReader>
 
 ScaleImageThread::ScaleImageThread(QObject *parent)
     : QThread(parent)
@@ -142,21 +143,34 @@ void ScaleImageThread::executeTask(const TaskData &task)
 QImage ScaleImageThread::scaleImage(const TaskData &task)
 {
     qDebug() << "scale image info:" << task.originalPath << " targetSize:" << task.targetSize;
-    QImage image;
-    image.load(task.originalPath);
+
+    QImageReader reader(task.originalPath);
+    if (!reader.canRead()) {
+        qWarning() << "Cannot read image:" << task.originalPath;
+        return QImage();
+    }
+
+    QSize originalSize = reader.size();
+    if (originalSize.isEmpty()) {
+        qWarning() << "Cannot get image size:" << task.originalPath;
+        return QImage();
+    }
+
+    QSize decodeSize = originalSize;
+    decodeSize.scale(task.targetSize, Qt::KeepAspectRatioByExpanding);
+    reader.setScaledSize(decodeSize);
+
+    QImage image = reader.read();
     if (image.isNull()) {
-        qWarning() << "load image failed:" << task.originalPath;
+        qWarning() << "load image failed:" << task.originalPath << reader.errorString();
         return image;
     }
 
-    QSize size = task.targetSize;
-    image = image.scaled(size, Qt::KeepAspectRatioByExpanding, Qt::FastTransformation);
-    image = image.copy(QRect((image.width() - size.width()) / 2,
-                             (image.height() - size.height()) / 2,
-                             size.width(),
-                             size.height()));
-
-    return image;
+    const QSize &size = task.targetSize;
+    return image.copy(QRect((image.width() - size.width()) / 2,
+                            (image.height() - size.height()) / 2,
+                            size.width(),
+                            size.height()));
 }
 
 QString ScaleImageThread::cacheImageToDisk(QImage &image, const TaskData &task, const QString &md5String)

src/plugin-qt/wallpapercache/wallpapercacheservice.cpp

@@ -201,10 +201,14 @@ QString WallpaperCacheService::saveImageFromFd(const QDBusUnixFileDescriptor &fd
     }
     originFile.close();
 
-    // Detect format and rename with extension
-    QImageReader reader(originPath);
-    QString format = reader.format();
-    QString destinationPath = originPath + QString(".%1").arg(format);
+    // Detect format using static method to avoid loading entire image
+    QByteArray format = QImageReader::imageFormat(originPath);
+    if (format.isEmpty()) {
+        qWarning() << "Failed to detect image format:" << originPath;
+        QFile::remove(originPath);
+        return QString();
+    }
+    QString destinationPath = originPath + QString(".%1").arg(QString::fromLatin1(format));
     QFile file(originPath);
     if (!file.rename(destinationPath)) {
         qWarning() << "originPath rename failed";