Students joining or reconnecting while the session is in question_closed
or finished previously got only a 'state' broadcast and nothing else,
leaving the SPA stuck on whatever was rendered before (typically the
join form's disabled state). Now send_student_snapshot replays the
question_closed message (and on finished, the session_ended payload)
so the SPA can render the reveal or final card immediately.
Mirrors the instructor-side reconnect fix in 22d1096.
8e8d5cfff0