feat(steps): hand pending root steps to a quickshell window

Notification balloons carried the GRUB and homepage root commands, but a
balloon cannot stay open while its commands are pasted into a root shell.
bin/udt-steps NAME writes a consumer's pending steps to
~/.cache/udt/steps/NAME and pokes quickshell's appearance shell, which shows
one tab per file on DP-3. Empty stdin clears the tab, so producers report
on every run and a consumer that caught up drops out by itself.

udt-grub and udt-homepage now route through it. install.sh's SDDM block
checks what is installed and reports only the steps still missing, rather
than printing the full install on every run. udt-homepage remembers the
last staged pair, since udt-accent now also runs on icon switches, which
change neither file and would re-raise the finish steps for nothing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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