feat: add search bar to bounties page (Closes #823)

[liufang88789-ui]

Adds a debounced search input to the /bounties page and wires it into the existing bounty query params so search works alongside the current status and language filters.

Highlights:

• search input visible on the bounties page
• 300ms debounce before querying
• clear button to reset search instantly
• search param passed together with existing status and skill filters
• improved empty-state copy for search results
• focused test coverage for debounce + clear behavior

Validation:

• npm test -- --run src/__tests__/bounty-search.test.tsx ✅

Note:

• the repo was missing tracked frontend/src/lib/animations.ts and frontend/src/lib/utils.ts modules even though many existing components import them; this PR includes minimal implementations so the current frontend code can resolve those imports consistently.

Closes #823

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.