Update loading skeletons

[obulat]

Fixes

Fixes #4829 by @fcoveram

Description

This PR updates the skeleton colors to match the updated designs.

It also fixes the number of columns when the sidebar is visible: the isSidebarVisible prop wasn't passed, so the number of columns was set the same when the sidebar was open, and the screen was narrower, as when it was closed.

It also moves some styles to Tailwind classes, and simplifies the Storybook story.

Testing Instructions

Open the browser devtools and set slower speed in the "Network" tab's throttling settings.
Try searching for something in Openverse. Use some new search terms to prevent loading from cache. You should see loading skeletons. The number of columns in "All content" view should match the number of columns in the results when the filters sidebar is open (In staging, you can see that there are more columns of the skeleton than of the search results when the sidebar is open)

Check the loading skeletons in Storybook, in both dark and light modes.
Emulate prefers reduced motion using the browser devtools, and see that the skeletons are not animated.

Checklist

• My pull request has a descriptive title (not a vague title likeUpdate index.md).
• My pull request targets the default branch of the repository (main) or a parent feature branch.
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.
• I ran the DAG documentation generator (ov just catalog/generate-docs for catalog
  PRs) or the media properties generator (ov just catalog/generate-docs media-props
  for the catalog or ov just api/generate-docs for the API) where applicable.

Developer Certificate of Origin

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[github-actions]

Full-stack documentation: https://docs.openverse.org/_preview/4982

Please note that GitHub pages takes a little time to deploy newly pushed code, if the links above don't work or you see old versions, wait 5 minutes and try again.

You can check the GitHub pages deployment action list to see the current status of the deployments.

[github-actions]

Latest k6 run output¹

$     ✓ status was 200

     checks.........................: 100.00% ✓ 8000      ✗ 0   
     data_received..................: 1.8 GB  8.5 MB/s
     data_sent......................: 1.0 MB  4.9 kB/s
     http_req_blocked...............: avg=7.65µs   min=1.32µs  med=2.44µs   max=3.48ms  p(90)=3.51µs   p(95)=3.83µs  
     http_req_connecting............: avg=4.78µs   min=0s      med=0s       max=3.42ms  p(90)=0s       p(95)=0s      
     http_req_duration..............: avg=555.58ms min=37.49ms med=500.69ms max=2.31s   p(90)=1.03s    p(95)=1.14s   
       { expected_response:true }...: avg=555.58ms min=37.49ms med=500.69ms max=2.31s   p(90)=1.03s    p(95)=1.14s   
   ✓ http_req_failed................: 0.00%   ✓ 0         ✗ 8000
     http_req_receiving.............: avg=150.63µs min=39.22µs med=109.92µs max=24.18ms p(90)=179.99µs p(95)=220.78µs
     http_req_sending...............: avg=21.63µs  min=6.64µs  med=19.55µs  max=3.08ms  p(90)=26.16µs  p(95)=32.85µs 
     http_req_tls_handshaking.......: avg=0s       min=0s      med=0s       max=0s      p(90)=0s       p(95)=0s      
     http_req_waiting...............: avg=555.41ms min=37.35ms med=500.54ms max=2.31s   p(90)=1.03s    p(95)=1.14s   
     http_reqs......................: 8000    37.260877/s
     iteration_duration.............: avg=3.71s    min=1.17s   med=3.26s    max=11.32s  p(90)=7.97s    p(95)=8.41s   
     iterations.....................: 1200    5.589132/s
     vus............................: 8       min=8       max=30
     vus_max........................: 30      min=30      max=30

Footnotes

1. This comment will automatically update with new output each time k6 runs for this PR ↩

[github-actions]

Playwright failure test results: https://github.com/WordPress/openverse/actions/runs/11064395080

It looks like some of the Playwright tests failed. If you made changes to the frontend UI without updating snapshots, this might be the cause. You can download zipped patches containing the updated snapshots alongside a general trace of the tests under the "Artifacts" section in the above page. They're named in the form *_snapshot_diff and *_test_results respectively.

You can read more on how to use these artifacts in the docs.

If the test is flaky, follow the flaky test triage procedure.

[openverse-bot]

Based on the medium urgency of this PR, the following reviewers are being gently reminded to review this PR:

@zackkrida
@dhruvkb
@fcoveram
This reminder is being automatically generated due to the urgency configuration.

Excluding weekend¹ days, this PR was ready for review 5 day(s) ago. PRs labelled with medium urgency are expected to be reviewed within 4 weekday(s)².

@obulat, if this PR is not ready for a review, please draft it to prevent reviewers from getting further unnecessary pings.

Footnotes

1. Specifically, Saturday and Sunday. ↩

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