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

[zp6]

Loading Skeleton Animations

Closes #827

Implementation

• Reusable Skeleton base component with shimmer animation
• BountyCardSkeleton — matches bounty card layout
• LeaderboardRowSkeleton — matches leaderboard rows
• ProfileSectionSkeleton — matches profile sections
• Uses Tailwind animate-pulse for shimmer effect
• Smooth transition to real content when loaded

Usage

{loading ? <BountyCardSkeleton /> : <BountyCard data={data} />}

Acceptance Criteria

• Skeletons visible on all pages during data loading
• Match the layout of actual content
• Smooth transition when data loads

Wallet: zp6