fix(mobile): use ContestCardSkeleton for Explore contest carousel loading state

[dylanjeffers]

Summary

• Swap the loading state of the Explore contest carousel from TrackCardSkeleton to ContestCardSkeleton so the placeholder matches the actual card shape.
• Wrap the skeleton with minWidth: 250 so its width matches the rendered ContestCard.

Test plan

• Open the mobile Explore screen with a slow/throttled network and confirm the contests carousel shows contest-shaped skeletons while loading.
• Once contests load, verify cards render correctly without layout shift from the skeleton width.

🤖 Generated with Claude Code

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