feat: add loading skeleton animations for all pages (#827)

[wingo-blue]

Summary

Add comprehensive loading skeleton animation system across all data-loading states.

New files

• Skeleton.tsx ??Base skeleton primitives (SkeletonBox, SkeletonLine, SkeletonAvatar, SkeletonBadge) + pre-built skeletons for bounty cards, leaderboard, profile sections, podium
• animations.ts ??Shared framer-motion animation variants (skeletonPulse, fadeIn, staggerContainer, etc.)
• utils.ts ??Utility functions (cn, formatCurrency, timeLeft, timeAgo, LANG_COLORS, etc.)

Modified

• BountyGrid.tsx ??Replaced basic gradient-bar loading with SkeletonBountyGrid matching BountyCard shape
• LeaderboardPage.tsx ??Replaced spinner with SkeletonPodium + SkeletonLeaderboardTable
• ProfileDashboard.tsx ??Replaced text 'Loading...' with SkeletonProfileSection

Key Features

• Shimmer animation via existing �nimate-shimmer Tailwind config
• Skeletons precisely match real content shapes (card layout, table rows, profile sections, podium)
• Framer-motion mounted for smooth appearance
• Drop-in replacements ??existing loading states simply swap the child component
• No new dependencies ??uses existing framer-motion and Tailwind

Closes #827

Wallet: A6YMFucCab3KvqLxHAtaUFxFUReSTPUg4dTxn1kQy7t