refactor(mpv): move ModernZ overrides into mpv.conf via script-opts-append

- drop defaults/etc/mpv/script-opts/modernz.conf (deploy timing conflict
  with mpv-modernz-git)
- put all ModernZ tweaks in mpv.conf: Catppuccin Lavender accent, scale
  0.75, title font 18, ontop_button off, autofit-larger 80%
- quote hex values to survive mpv's mid-line # comment parsing
- niri window-rule opens mpv floating
- doctor no longer compares mpv configs (now directly owned by pkg)

defaults/xdg/niri/config.kdl

@@ -116,6 +116,11 @@ window-rule {
     open-floating true
 }
 
+window-rule {
+    match app-id=r#"^mpv$"#
+    open-floating true
+}
+
 window-rule {
     geometry-corner-radius 4
     clip-to-geometry true