feat: add loading skeleton animations (Closes #827)

[liufang88789-ui]

Adds reusable shimmer skeleton components and uses them across the main loading states that currently show generic spinners/placeholders.

Implemented skeletons for:

• bounty cards / bounty grid
• featured bounties section
• leaderboard rows
• profile summary and profile bounty rows

Highlights:

• reusable skeleton primitives with shimmer animation
• loading states now visually match final layout more closely
• smoother perceived transition from loading to loaded content
• focused test coverage for skeleton rendering

Validation:

• npm test -- --run src/__tests__/skeletons.test.tsx ✅

Closes #827

Wallet: 7UqBdYyy9LG59Un6yzjAW8HPcTC4J63B9cZxBHWhhHsg

[liufang88789-ui]

Noticed the AI Code Review workflow failed with a GitHub API rate-limit error during trigger-review, rather than from a code-level review result. When rate limits recover, could you please rerun the review workflow for this PR? Thanks.