Address review feedback: init-time model switches (#reconcileModeFromSession,
#enterPlanMode for plan.defaultOnStartup) fire model_changed before the
subscription exists, leaving the banner stale for those cases too.
Adds a catch-up #updateWelcomeModel() call right after the subscription is
installed, mirroring how the sessionName listener is registered before
those same init steps (line 1094-1095 comment).
Also adds a regression test for the catch-up path and comments tying the
component test to the wiring gap it covers.
jacobleft
272a0e8461