fix: prevent lock screen during screensaver preview (#119)

Fixed an issue where the lock screen would be triggered during
screensaver preview mode. Added a check for m_previewing flag to ensure
lock screen is not activated when previewing screensavers.

The previous logic would trigger lock screen when screensaver stopped if
certain conditions were met, including when the screensaver was awakened
by the window itself. However, this didn't account for preview mode
where users are just testing screensavers and shouldn't be locked out.
Added !m_previewing condition to prevent lock screen activation during
preview sessions.

Log: Fixed screensaver preview triggering unintended lock screen

Influence:
1. Test screensaver preview functionality - ensure preview works without
locking screen
2. Test normal screensaver timeout - ensure lock screen still activates
after timeout
3. Test manual lock screen activation - ensure it still works when
requested
4. Verify screensaver stops correctly in both preview and normal modes
5. Test awakening from screensaver - ensure lock screen activates only
when appropriate

fix: 修复屏保预览时触发锁屏的问题

修复了在屏保预览模式下会触发锁屏的问题。添加了对 m_previewing 标志的检
查，确保在预览屏保时不会激活锁屏。

之前的逻辑会在屏保停止时触发锁屏，前提是满足某些条件，包括当屏保被窗口自
身唤醒时。但这没有考虑到预览模式，用户只是测试屏保效果，不应该被锁定。添
加了 !m_previewing 条件来防止在预览会话期间激活锁屏。

Log: 修复屏保预览时意外触发锁屏的问题

Influence:
1. 测试屏保预览功能 - 确保预览工作正常且不会锁定屏幕
2. 测试正常屏保超时 - 确保超时后锁屏仍然激活
3. 测试手动锁屏激活 - 确保请求锁屏时仍然正常工作
4. 验证屏保在预览和正常模式下都能正确停止
5. 测试从屏保唤醒 - 确保只在适当时机激活锁屏

PMS: BUG-345823

Author: robertkill

src/dbusscreensaver.cpp

@@ -417,8 +417,8 @@ void DBusScreenSaver::Stop(bool lock)
     if (m_windowMap.isEmpty())
         return;
 
-    // 只在由窗口自己唤醒时才会触发锁屏
-    if (m_lockScreenAtAwake && !m_lockScreenTimer.isActive()
+    // 只在由窗口自己唤醒时才会触发锁屏，预览模式下不触发锁屏
+    if (m_lockScreenAtAwake && !m_lockScreenTimer.isActive() && !m_previewing
             && (lock || qobject_cast<ScreenSaverWindow*>(sender()))) {
         auto ver = DSysInfo::majorVersion().toInt();
         QString lockfontService = "com.deepin.dde.lockFront";