Replace loading spinners with shimmer skeletons

[sepulchralvoid666]

Closes #827

Wallet: 9PmjWFwM2mdnjBNjJyfNMTWGCnGAKFpywsLF9xcRF8PK

[nagiexplorer88]

This still misses the "all pages during data loading" acceptance criterion for #827. The new skeletons replace loading UI in BountyGrid, BountyDetailPage, LeaderboardPage, and one ProfileDashboard tab, but the home page still has a data-loading state that uses the old placeholder.

FeaturedBounties.tsx calls useBounties({ limit: 4, status: "open" }), and while isLoading it still renders four plain animate-pulse blocks rather than the new shimmer skeleton components. Since FeaturedBounties is part of the home page, reviewers will still see a non-matching loading placeholder there instead of the requested skeleton layout.

A small fix would be to reuse BountyCardSkeleton in FeaturedBounties too, so the home page bounty-loading state matches the actual cards and the new skeleton system.

[nagiexplorer88]

Confirmed. The follow-up commit now imports BountyCardSkeleton in FeaturedBounties.tsx and uses it for the home page loading state instead of the old plain animate-pulse placeholders:

{Array.from({ length: 4 }).map((_, i) => (
  <BountyCardSkeleton key={i} />
))}

That covers the home page loading path I called out, so the skeleton system is now applied there as well.