Fix dimming animation flicker

[smchase]

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Screen.Recording.2025-12-24.at.3.17.56.PM.mov

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Description

Fixes #6179, see it for context on the issue.

This bug is due to the fact that the parent message dimming animation is composed of a fadeIn and separate async fadeOut. When we tap in quick succession we call dimmerLayer.removeAllAnimations() to remove any old animation, but this doesn't include the fadeOut if the async hasn't fired yet. This means we end up with multiple old async fadeOuts causing the parent message to flicker once the tapping stops.

The fix is to make this into one sync animation that contains the fade in and fade out. This way when the link is tapped again we clear the entire old animation including the fade out. This seems simpler and cleaner than tracking the async queue and clearing from there, and more logically correct to treat the animation as atomic.

[smchase]

first PR here so apologies if I missed anything, but hopefully this is a small enough change with enough context on the issue to be easy to review :)