When the local user sends a particle while positioned on the last one in a
stream, advance playback to the newly created particle so they stay caught up
and the stream isn't left in an unseen state. If they're browsing earlier
particles, their position is left untouched.
The mobile reducer only auto-advanced on PARTICLE_ADDED when status was
already "ended", so sending while sitting on (but not finished with) the last
particle left it unseen. This threads an onParticleCreated callback from
ComposeDock's default send paths up to StreamView, which follows the particle
via goToParticle only when currentIndex is the last slot — mirroring desktop's
StreamView.handleParticleCreated.
Claude