Reader: fix Recent feed post loading placeholder animation

[holdercp]

Proposed Changes

• Uses a more specific name for the animation the Recent feed loading placeholder to prevent collisions

Before

Screen.Recording.2025-01-15.at.5.52.29.PM.mov

After

Screen.Recording.2025-01-15.at.5.53.19.PM.mov

Why are these changes being made?

• As I've been working on Reader, I kept noticing the post loading placeholder would look weird (see before video) every once and awhile but I could never reliably reproduce it.
• I discovered this issue was a keyframe name collision (pulse) that happened when another stylesheet was loaded async. Because it came later in the cascade, it took precedence and the loading animation looked wonky.
• The stylesheet is loaded when the Sites masterbar button is hovered, presumably to pre-load the styles for the sites page.

Testing Instructions

• Navigate to /read
• Click on posts in the Recent feed to trigger the post loading placeholder
• Notice the loading animation
• Hover the Sites button in the masterbar
• Trigger the post loading placeholder again and notice the animation. It should look as expected.
• Do these steps on production or staging to see the bug if desired

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
  • For UI changes, have we tested the change in various languages (for example, ES, PT, FR, or DE)? The length of text and words vary significantly between languages.
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

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[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.