fix plasma: disable stylix qt target and set widgetStyle=Breeze

Stylix's qt target sets QT_STYLE_OVERRIDE=kvantum and writes qt6ct/qt5ct
configs with style=kvantum. plasmashell/KWin crash with a fatal
'module kvantum is not installed' QML error because the kvantum Qt style
plugin is not available in the Plasma session.

- stylix: targets.qt.enable = false (stops QT_STYLE_OVERRIDE=kvantum)
- plasma: remove kvantum package, add widgetStyle=Breeze as belt-and-suspenders

modules/home/stylix/default.nix

@@ -101,6 +101,14 @@ in
         useWallpaper = false;
       };
       kde.enable = false;
+      # Disable Stylix Qt theming (qt6ct/qt5ct + kvantum). When enabled, Stylix
+      # sets QT_STYLE_OVERRIDE=kvantum and writes qt6ct/qt5ct configs with
+      # style=kvantum. Plasma's plasmashell/KWin crash with a fatal
+      # "module kvantum is not installed" QML error because the kvantum Qt style
+      # plugin is not available. The Plasma home module sets widgetStyle=Breeze
+      # to counteract this, but disabling the source of the problem here is
+      # simpler and avoids the issue for all desktops.
+      qt.enable = false;
       firefox = {
         enable = false;
         profileNames = [