feat: add loading skeleton animations (#827)

[CHY9213]

Loading Skeleton Animations

Shimmer loading skeletons for all data-loading states across the site.

Components

Component	Usage
Skeleton	Base shimmer block (any shape via className)
BountyCardSkeleton	Matches bounty card layout
LeaderboardRowSkeleton	Matches leaderboard rows
ProfileSectionSkeleton	Matches profile sections

Features

• Shimmer animation via Tailwind animate-pulse
• Match layout of actual content
• Smooth transition when data loads
• Accessible (aria-hidden="true")
• Dark mode compatible

Usage

import { BountyCardSkeleton } from '../components/ui/Skeleton';

{loading && Array.from({ length: 6 }).map((_, i) => (
  <BountyCardSkeleton key={i} />
))}

Closes #827

Wallet: 2oWEBzKDQo9KYxAFYMPrR8QiwKJia4vbSXrbV4ep7eTz