The idleSpriteWrapperStyle was using `keyframes.toString()` inside a JS
template literal, so Emotion's serializer hit the string-handling branch
and never injected the @keyframes block — the animation silently did
nothing. Switch to direct Keyframes-object interpolation inside css\`...\`
so Emotion registers the rule and returns the animation name.
Also add 'wheel' to ACTIVITY_EVENTS so desktop mouse-wheel scrolling on
the inner scrollable content area resets the idle timer (the existing
'scroll' listener on window only catches mobile/touch scroll).
Update the source plan doc to reflect the corrected idiom so future
implementers don't repeat the bug.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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