Install what the default-application roles are curated to

Seeding is deliberately conservative: a role whose preferred applications
are all missing is left alone rather than forced. That is right, and it
is also silent -- so a curated handler nobody installs presents as the
machine quietly going back to deciding defaults by installation order,
which is the problem seeding exists to fix. None of Loupe, Papers,
Decibels, Nautilus, mpv or sushi was declared anywhere.

Preview works, on org.gnome.NautilusPreviewer2 rather than the interface
its bus name suggests, but it opened tiled -- shoving the file manager
aside for something meant to be an overlay -- so it gets a float and
center rule sized to leave the file underneath visible.

xdg-utils and desktop-file-utils were undeclared too; seeding from
link-dotfiles started calling them, and the dependency contract said so.

Claude-Session: https://claude.ai/code/session_01BRvzt4H8XXLPVH5MyYdk9L

tests/quickshell/declared-dependencies-contract.sh

@@ -56,6 +56,8 @@ package_for() {
         notify-send)    printf 'libnotify' ;;
         wl-copy|wl-paste) printf 'wl-clipboard' ;;
         rg)             printf 'ripgrep' ;;
+        xdg-mime|xdg-settings|xdg-open) printf 'xdg-utils' ;;
+        update-desktop-database|desktop-file-validate) printf 'desktop-file-utils' ;;
         python3)        printf 'python3' ;;
         *)              printf '%s' "$1" ;;
     esac