Let the invitation terminals actually open

Clicking the Voxtype or fingerprint invitation did nothing. The launcher
execs setsid, which forks because the transient unit's main process is
already a process group leader, so the unit exits within milliseconds and
systemd's default control-group kill took the still-starting terminal down
with it. Run those units with KillMode=process instead.

The invitation tests had also drifted from the two-unit design and were
failing; teach the systemd-run mock to run the command it is given.

install/user/first-run/install-voxtype.hook

@@ -8,7 +8,10 @@ show_invitation() {
     # can exit right after the click. If it stayed alive for the life of the
     # install terminal, the installer's omarchy-restart-shell would re-trigger
     # this still-running *-invitation unit and pop the toast a second time.
-    systemd-run --user --collect --quiet \
+    # KillMode=process because the launcher's setsid forks and returns, so the
+    # unit's main process exits within milliseconds. The default control-group
+    # kill would take the terminal down with it before it ever appears.
+    systemd-run --user --collect --quiet -p KillMode=process \
       --unit=omarchy-voxtype-install \
       omarchy-launch-floating-terminal-with-presentation omarchy-voxtype-install
   fi

install/user/first-run/setup-fingerprint.hook

@@ -8,7 +8,10 @@ show_invitation() {
     # exit right after the click. If it stayed alive for the life of the setup
     # terminal, the setup's omarchy-restart-shell would re-trigger this
     # still-running *-invitation unit and pop the toast a second time.
-    systemd-run --user --collect --quiet \
+    # KillMode=process because the launcher's setsid forks and returns, so the
+    # unit's main process exits within milliseconds. The default control-group
+    # kill would take the terminal down with it before it ever appears.
+    systemd-run --user --collect --quiet -p KillMode=process \
       --unit=omarchy-setup-security-fingerprint \
       omarchy-launch-floating-terminal-with-presentation omarchy-setup-security-fingerprint
   fi