Add bounty marketplace search bar

[snidova1]

Summary

• Add a debounced search input to the bounty marketplace filters
• Send the free-text query through the bounties list API as q
• Apply client-side matching across title, description, category, org/repo, issue URLs, and skills for loaded results
• Add shared frontend helper modules needed by the current frontend imports so the production build can compile
• Add API coverage for the new bounty search query parameter

Fixes #823

Validation

• cd frontend && npm test -- --run src/__tests__/bounties-api.test.ts src/__tests__/apiClient.test.ts
• cd frontend && npm run build

Note: the full frontend test suite still contains existing broken suites that reference files/modules not present in the repo (admin, tokenomics, bounties legacy components, Playwright e2e, etc.); the targeted tests above pass and the production build succeeds.