feat: Loading Skeleton Animations (Bounty #827)

[dinnar1407-code]

Summary

Comprehensive loading skeleton system with shimmer animations for all data-loading states.

Components

• Skeleton — Base shimmer block (configurable width, height, circle mode)
• BountyCardSkeleton — Matches bounty card layout (title, description, tags, reward)
• LeaderboardRowSkeleton — Matches leaderboard row (rank, avatar, name, score)
• ProfileSkeleton — Matches profile header (avatar, name, stats grid)
• BountyGridSkeleton — Grid of bounty card skeletons (configurable count)
• LeaderboardSkeleton — List of leaderboard row skeletons

Features

• Shimmer gradient animation (1.5s infinite)
• Matches actual content dimensions for seamless transition
• aria-hidden='true' on all skeletons
• Drop-in replacements for loading states

Acceptance Criteria

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

Closes #827