feat: add freelancer discovery page

[OkeyAmy]

Close #117

Summary

• Added a responsive freelancer discovery page at /freelancers with search, multi-select skill filters, minimum rating filters, pagination, loading cards, and empty/error states.
• Added a dynamic /api/freelancers endpoint that applies keyword, skill, rating, and pagination criteria together.
• Added shared mock freelancer listing data and skill metadata for the discovery endpoint and UI.
• Added desktop and mobile navbar links to the freelancer browse page.

Testing

• npm run lint
• npm run build
• npm test

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