feat: expand loading skeleton animations across key pages

[alchip]

Summary

• expand shimmer skeleton coverage to additional loading states
• improve Home FeaturedBounties loading cards with structured shimmer placeholders
• add leaderboard page skeleton layout (podium + table placeholders)
• replace profile "Loading..." text with stacked shimmer rows in MyBountiesTab

Why

Issue #827 requests loading skeleton animations across all major data-loading surfaces with smooth transition to real content.

Acceptance mapping

• Skeletons visible on key pages during loading
• Skeletons match actual content layout patterns
• Uses existing shimmer animation style for consistency

Closes #827

Validation

• Manual verification by inspecting all touched loading branches/components
• npm run build in frontend remains blocked by pre-existing missing module paths (lib/animations, lib/utils) unrelated to this PR

Wallet

Wallet: 74jSPYaxEJcGEuW4jr6bqQdg2iuLChyLEuEmC8QhcQVW