feat: adjust notification center styling and opacity

1. Modified opacity calculation to enforce a minimum of 0.4 (40%) when
retrieving from the appearance applet, preventing the notification
center from becoming too transparent
2. Added dynamic top margin to the notification center area that only
appears when there are notifications in the staging area, improving
visual spacing and layout
3. The changes ensure better readability and visual hierarchy in the
notification interface

Log: Improved notification center visual appearance with better opacity
control and spacing

Influence:
1. Test notification center appearance with different system
transparency settings
2. Verify that opacity never drops below 40% even when system
transparency is set lower
3. Test notification display with and without notifications in the
staging area
4. Check layout spacing when staging area contains notifications versus
when empty
5. Verify overall visual consistency and readability

feat: 调整通知中心样式和透明度

1. 修改透明度计算，从外观小程序获取值时强制最小为0.4（40%），防止通知中
心变得过于透明
2. 为通知中心区域添加动态上边距，仅当暂存区域有通知时显示，改善视觉间距
和布局
3. 这些更改确保通知界面具有更好的可读性和视觉层次结构

Log: 改进通知中心视觉效果，提供更好的透明度控制和间距

Influence:
1. 测试不同系统透明度设置下的通知中心外观
2. 验证即使系统透明度设置更低时，透明度也不会低于40%
3. 测试暂存区域有通知和没有通知时的显示效果
4. 检查暂存区域包含通知与空置时的布局间距
5. 验证整体视觉一致性和可读性

PMS: BUG-354851

Author: 18202781743

panels/notification/center/package/main.qml

@@ -142,7 +142,7 @@ Window {
         var appearance = DS.applet("org.deepin.ds.dde-appearance")
         if (!appearance || appearance.opacity < 0)
             return fallback
-        return appearance.opacity
+        return Math.max(appearance.opacity, 0.4)
     }
 
     // only add blendColor effect when DWindow.enableBlurWindow is true,
@@ -212,6 +212,7 @@ Window {
             id: notifyCenter
             anchors {
                 top: notifyStaging.bottom
+                topMargin: notifyStaging.viewCount > 0 ? contentPadding : 0
                 left: parent.left
                 right: parent.right
                 bottom: parent.bottom