perf(ui): convert message pairing to O(n) forward pass in chain-of-thought

[ishaanxgupta]

Summary

Optimizes the message pairing algorithm in the ChainOfThought component.

Previously, for every user message in the chat thread, the component created a new sliced array and iterated forward with .find() to locate the next assistant message. This resulted in an $O(n^2)$ time complexity and repeated garbage collection, which becomes noticeably sluggish on long conversations.

Changes

• Removed the .slice(i + 1).find(...) nested iteration inside the for loop.
• Introduced a lastUserPair state tracker to map the first subsequent assistant message directly to the most recent user message in a single $O(n)$ forward pass.