wingpanel: KillMode=process so restarting the panel keeps its apps (#662)

Apps opened from the panel live in pantheon-wingpanel.service's cgroup;
with the default control-group kill mode, any panel restart (including
install-network-throughput.sh and activate-t3-panel.sh) also killed them,
e.g. Messages and transmission-daemon. Same fix as the dock drop-in.

Verified with a user drop-in: restarting the panel freed 1.6 GiB of
stale heap and Messages and transmission-daemon kept running.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: techmore

mseries/pantheon-rust-compositor/session/usr/lib/systemd/user/pantheon-wingpanel.service

@@ -17,3 +17,6 @@ ExecStart=/usr/bin/io.elementary.wingpanel
 Restart=on-failure
 RestartSec=2s
 TimeoutStopSec=5s
+# Apps opened from the panel (Applications menu, indicators) live in this unit's
+# cgroup. Restarting or replacing the panel must end only the panel, not them.
+KillMode=process