[MEDIUM] Build the Browse Jobs page with mock data

[spiffamani]

Summary

Implement frontend/src/pages/BrowseJobs.tsx using the JobCard component. Use mock data for now — real blockchain data comes later.

Acceptance Criteria

• Page renders a grid of at least 6 mock job listings using JobCard
• Filter bar at top with category filter (All, Design, Dev, Writing)
• Filtering works client-side
• Empty state shown when no jobs match filter
• Mobile responsive grid (1 col mobile, 2 col tablet, 3 col desktop)