[FE-29] Create member public profile page

[yusuftomilola]

Overview

Build a public-facing member profile page that shows a member's badges, activity stats, and check-in streak. All work goes in frontend/sandbox/.

Acceptance Criteria

• Page at frontend/sandbox/members/[id]/page.tsx
• Shows: profile photo, name, member since date, streak count, total check-ins, badges earned
• Badges displayed as visual chips with badge name
• Activity stats shown using the StatCard component
• "Member since" formatted as "March 2024"
• 404 page shown if member not found
• Uses mock data if no API connected