Replace dunst with swaync for fractional scaling support

Dunst lacks wp_fractional_scale_v1, causing jagged font rendering
on external monitors in mixed-DPI setups (laptop 2.5x + externals 1x).
swaync (GTK4) handles fractional scaling natively.

Style based on catppuccin/swaync with Lavender accent.

config/user_configuration.json

@@ -46,7 +46,7 @@
         "niri",
         "waybar",
         "foot",
-        "dunst",
+        "swaync",
         "polkit-gnome",
         "stow"
     ],