fix(planet/hm/qt): use gtk3 platform theme

Used to be `gtk`, which seems to be GTK 2. Since the `catppuccin-gtk`
package does not work with GTK 2 any more, Qt programs were not themed
correctly.

planet/modules/home-manager/qt/default.nix

@@ -20,7 +20,7 @@
     mkIf cfg.enable {
       qt = {
         enable = true;
-        platformTheme.name = "gtk";
+        platformTheme.name = "gtk3";
       };
     };
 }