fix(web): replace hasOpenedDiff useState/useEffect latch with useRef

[Mergemat]

Replaces a useState + useEffect latch for hasOpenedDiff with a useRef written inline during render.

What changed: removed the effect that called setHasOpenedDiff(true) when diffOpen was true. The ref is initialized to diffOpen and set inline: if (diffOpen) hasOpenedDiffRef.current = true.

Why: calling setState inside a useEffect schedules an extra render after the initial one completes. The value is never needed to trigger a re-render — only to gate rendering of diff content — so a ref is the right tool.

Note

Replace hasOpenedDiff state latch with a ref in ChatThreadRouteView

Replaces a useState/useEffect pattern with a useRef to track whether the diff panel has been opened. The ref is initialized to the current diffOpen value and set synchronously when diffOpen is true, removing an unnecessary render cycle caused by the prior effect.

^{Macroscope summarized e07e402.}

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