Make chat effects more deterministic

Currently sending multiple emojis which would trigger multiple effects only triggers one on the sender side, but the receiver sees multiple. This is likely due to the handling being different - we should move all the effect processing into a single place to make it deterministic.

We should also probably decide if its a feature or a bug to overlap effects.

Start by reproducing the multiple-emoji case on both the sender and receiver sides, then trace where each side processes chat effects. Identify the separate processing paths and determine how overlapping effects should behave. Done means sender and receiver handle the same effects deterministically, with the overlap behavior explicitly decided.

Written by the indexing model from the issue text.