feat: add debounced search bar for bounties page

[alchip]

Summary

• add search input to bounties page above filter pills
• implement 250ms debounced query updates for smooth near-real-time filtering
• filter bounties by title, description, org/repo, issue URL, and skills
• add clear (X) control to reset search quickly
• improve empty-state messaging for search misses

Why

Issue #823 requests a bounties-page search bar that works with existing filters and supports practical matching fields.

Acceptance mapping

• Search bar visible on /bounties
• Typing filters bounties in near real-time (debounced)
• Works alongside existing status + language filters
• Clear button resets search

Closes #823

Validation

• Manual verification of filter interaction and search UX in BountyGrid
• npm run build in frontend currently blocked by pre-existing missing module paths (lib/animations, lib/utils) unrelated to this PR

Wallet

Wallet: 74jSPYaxEJcGEuW4jr6bqQdg2iuLChyLEuEmC8QhcQVW