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

[liqiuniu]

Summary

Adds professional shimmer loading skeletons for all data-loading states across the site. Addresses Bounty #827.

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Components Added

` (new file)

A comprehensive skeleton component library:

Component	Description
`	Base shimmer block with gradient animation
`	Matches BountyCard layout exactly
`	Single leaderboard row skeleton
`	Full table with header + rows
`	Top 3 podium with varying heights
`	Bounty detail page loading state
`	Configurable grid (default/featured)

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Pages Updated

Page	Change
`	Uses ` instead of simple pulse boxes
`	Shows + while loading
`	Uses ` with featured column layout
`	Shows ` while fetching bounty
`	Skeleton rows in My Bounties tab

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Features

• Shimmer animation - Smooth gradient sweep using existing ` CSS
• Layout matching - Skeletons match actual component dimensions
• Smooth transition - Content appears naturally after load
• Configurable - ` prop for different grid layouts

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Acceptance Criteria

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

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Testing

1. Navigate to any page (Bounties, Leaderboard, Profile)
2. Observe skeleton placeholders during API fetch
3. Content smoothly replaces skeleton when loaded

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Related

Bounty #827 鈥?#827