feat(fe): add contest leaderboard loading screen

[ojongii]

Description

Contest Leaderboard 탭에서 데이터 로딩 중 빈 화면 대신 스켈레톤을 보여줍니다.

Draft 풀리고 나서 리뷰해주세요!
#3672 머지 후 수정할 예정입니다.

이제 리뷰해주셔도 됩니다!

Additional context

closes TAS-2823

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Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines
• Read the Contributing Guidelines and follow the Commit Convention
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

Summary by CodeRabbit

• UX Improvements
  • Added a loading placeholder for the contest leaderboard, including headers, controls, table headings, and leaderboard rows.
  • Improved loading behavior so users see a consistent skeleton while leaderboard data is being retrieved.

[skkuding-bot]

✅ Syncing Preview App Succeeded

Application: frontend
Revision: 0f80bba62f05dc2d34bc438d7ed00d638f4c565a
Health Status: Healthy

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[skkuding-bot]

❗ Syncing Preview App Failed

Application: frontend
Revision: t2823-add-contest-leaderboard-loading-screen
Health Status: Healthy

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[coderabbitai]

🚥 Pre-merge checks | ✅ 2 | ❌ 3

❌ Failed checks (3 warnings)

Check name	Status	Explanation	Resolution
Linked Issues check	⚠️ Warning	The pull request adds a contest leaderboard loading skeleton, but linked issue #123 requires a workbook progress card with title, header, description, type, total, and completed values. The pull reque…	Implement the progress card component described in issue #123, or link the pull request to the issue that covers the contest leaderboard loading screen.
Out of Scope Changes check	⚠️ Warning	The leaderboard loading skeleton changes are unrelated to the linked issue #123, which concerns a workbook progress card component.	Remove the leaderboard loading changes from this pull request, or update the linked issue to the correct leaderboard loading-screen objective.
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: adding a loading screen for the contest leaderboard.

Full details: Linked Issues check

Explanation

The pull request adds a contest leaderboard loading skeleton, but linked issue #123 requires a workbook progress card with title, header, description, type, total, and completed values. The pull request does not address that objective.

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_{Comment @coderabbitai help to get the list of available commands.}

[skkuding-bot]

✅ Syncing Preview App Succeeded

Application: frontend
Revision: 70de0828b0282cbe8997e71dfd8fcc6a30ce5f61
Health Status: Healthy

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[skkuding-bot]

✅ Syncing Preview App Succeeded

Application: frontend
Revision: 46f1d866cad91442526b13958f72c07492243084
Health Status: Healthy

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[howso245-dot]

LGTM ! 수고하샸습니다

[skkuding-bot]

✅ Syncing Preview App Succeeded

Application: frontend
Revision: 1bd204e8bcbc62b7191e5d7f88e5ef577b0c69a0
Health Status: Healthy

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