Record that the X composer was opened

Instrumentation for the top of the shoutout funnel, and nothing else — it awards no points and does not move tasks.shoutout, which still requires the post to come back through /waitlist/tasks/shoutout and verify. Without this the click reached GA4 only, so the drop-off between opening the composer and submitting a link was unmeasurable. The timestamp is written on the FIRST click and never moved, so a re-click cannot migrate a click into a later reporting window. Callers should fire this without awaiting it: a failure here must never stop the composer from opening.