feat: apply cursor theme via GtkSettings instead of XCURSOR_THEME env (v0.9.0)

GTK4 under greetd does not honour XCURSOR_THEME reliably — the env-prefix
hack in /etc/greetd/config.toml only reached the wlroots pointer in cage,
while GTK widgets kept using the default fallback cursor. Mirror the
existing gtk-theme handling: new cursor-theme + cursor-size fields in the
[appearance] section, applied via gtk::Settings::set_gtk_cursor_theme_*.
Keeps the fix scoped to the greeter, no system-wide GTK config changes.

config/moongreet.toml

@@ -8,3 +8,8 @@ background = "/usr/share/backgrounds/wallpaper.jpg"
 # GTK theme name — must match a directory in /usr/share/themes/
 # Required because GTK4 under greetd does not reliably read settings.ini
 gtk-theme = "Colloid-Grey-Dark-Catppuccin"
+
+# Cursor theme name — must match a directory in /usr/share/icons/
+# GTK4 under greetd does not honour XCURSOR_THEME, so set it here.
+cursor-theme = "Sweet-cursors"
+cursor-size = 24