feat: add moonarch-waybar config merger wrapper

Waybar's include directive cannot merge arrays, making per-machine
module customization impossible without duplicating the entire config.

moonarch-waybar merges an optional ~/.config/waybar/userconfig with
the system config, supporting prepend/append on module arrays and
object merge for module definitions. Generates user style.css with
@import of system styles on first run.

System waybar config converted from JSONC to valid JSON for jq
compatibility. Niri startup and hotkey updated to use the wrapper.

defaults/xdg/niri/config.kdl

@@ -79,7 +79,7 @@ layout {
 
 // xwayland-satellite is managed automatically since niri 25.08
 // kanshi is managed via systemd user service (kanshi.service)
-spawn-at-startup "waybar"
+spawn-at-startup "moonarch-waybar"
 spawn-at-startup "swaync"
 spawn-at-startup "/usr/lib/polkit-gnome/polkit-gnome-authentication-agent-1"
 spawn-at-startup "nm-applet" "--indicator"
@@ -126,7 +126,7 @@ binds {
 
     Super+C hotkey-overlay-title=null { spawn "walker" "-s" "clipboard"; }
 
-    Alt+W { spawn-sh "killall waybar && waybar &"; }
+    Alt+W { spawn-sh "killall waybar && moonarch-waybar &"; }
 
     Super+E { spawn-sh "xdg-open ~"; }