Detach the pending-migrations toast from the notifier

omarchy-migrate-notify handed its notification to `systemd-run --scope`,
which is synchronous: the calling process becomes the payload, so
systemd-run does not return until the toast has been answered -- and if
the toast is clicked, not until the migration terminal it opens has been
closed.

omarchy-migrate-notify.service is Type=oneshot, which defaults to
TimeoutStartUSec=infinity, so the unit sat in activating for exactly that
long. On a machine that had not yet picked up c7e327b0, which orders the
notifier after graphical-session.target, that held the target open --
and wayland-wm-app-daemon.service is ordered after the same target, so it
never started. Every keybinding goes through uwsm-app, which waits ten
seconds for that daemon's pipes and then reports "App failure -- Timed
out waiting for pipes!" instead of launching anything. Keybindings were
dead for two minutes and seventeen seconds, until a pacman hook ran
`systemctl reload user@*.service`, whose re-exec of the user manager
broke the scope's bus connection and let the oneshot finish. That also
made systemd-run exit non-zero, so the notifier fell through to the
terminal fallback meant for having no user manager at all, and printed
the migration list into the journal.

Hand the notification to a transient service instead. systemd-run
returns once the unit has started rather than once the payload is done,
so the oneshot completes in milliseconds and nothing ordered after the
target waits on a toast. Put it in background-graphical.slice, which
systemd ships with PartOf=graphical-session.target, so an unanswered
toast ends at logout rather than outliving the session.

The terminal a clicked toast opens is unaffected: uwsm-app re-registers
it into app-graphical.slice, outside this unit's cgroup.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

bin/omarchy-migrate-notify

@@ -40,11 +40,14 @@ if update_in_progress; then
   exit 0
 fi
 
+# A transient service rather than a scope, which would block here until the
+# toast was answered and keep this oneshot activating for that whole time. The
+# graphical slice ends an unanswered toast at logout.
 unit="omarchy-migrations-notification-$(date +%Y%m%d%H%M%S)"
-systemd-run --user --scope --unit="$unit" bash -lc "$notify_command" >/dev/null 2>&1 && exit 0
+systemd-run --user --collect --slice=background-graphical.slice --unit="$unit" bash -lc "$notify_command" >/dev/null 2>&1 && exit 0
 
-# Reached when there is no user manager to run the scope under, such as a
-# non-graphical shell, so fall back to telling the user in the terminal.
+# Reached when the notification could not be handed off, so fall back to telling
+# the user in the terminal.
 print_pending_migrations() {
   echo "Omarchy has pending migrations. Run omarchy-migrate in a terminal to apply them:"
   while IFS= read -r migration; do