fix: define the shimmer animation start frame

[fredrikblau]

Description

The shared shimmer animation only declares its ending transform and relies on a utility class to provide the initial state. That implicit starting frame is not consistently retained by browsers, and the loading indicator can fail to animate correctly.

Define the 0% transform in the keyframes so the shimmer has an explicit, portable start state.

Related Issue

• Fixes File list: one IntersectionObserver and one shimmer animation per item #3169

How Has This Been Tested?

• test environment: Node.js 26.7.0, pnpm 11.24.0
• test case 1: pnpm lint
• test case 2: pnpm format:check

Types of changes

• Bugfix
• Enhancement (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt (improving code quality without changing functionality)
• Tests (adding or updating tests)
• Documentation (updates to the documentation, readme, or changelog)
• Maintenance (updates to the build process or auxiliary tools and libraries)

[JammingBen]

It seems you also included this in #3264, so I'm closing here.