fix: add integration test for holdings page loading skeleton (#627)

[sojetunde8]

Closes #627

Summary

• Adds integration test confirming skeleton placeholders render while the holdings query is in flight
• Adds CreatorHoldingsSkeleton and CreatorHoldingsListSkeleton components for the holdings loading state
• Verifies skeletons are replaced by real holding entries once data arrives
• Asserts no skeleton elements remain after data loads

Testing

• Ran both new integration tests and existing holdings tests — all 5 pass

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