style(qt): a touch more blur on the Wallpaper background

The Wallpaper mode's blur goes from 0.6 to 0.75 of MultiEffect's 64 px
maximum (about 38 to 48 px), so the picture behind the window stays
recognisable while competing even less with the text.

cc

crates/qbz-qt/qml/shell/AppShell.qml

@@ -140,7 +140,7 @@ Rectangle {
         visible: root.wallpaperModeOn
         source: root.wallpaperModeOn ? QbzShell.wallpaperUrl : ""
         hostWindow: root.hostWindow
-        blur: 0.6
+        blur: 0.75
         dim: theme.isDark ? QbzShell.ambientDim : 0.0
     }
     Rectangle {