[FE-54] Build admin overview dashboard page

[mftee]

Overview

Build an admin dashboard overview page with key metrics, recent activity, and quick action cards. All work goes in frontend/sandbox/.

Acceptance Criteria

• Page at frontend/sandbox/admin/page.tsx
• Top row: 4 stat cards (Total Members, Active Bookings Today, Revenue This Month, Workspaces Available) using the StatCard component
• Middle row: line chart (bookings per day, last 30 days) and pie chart (bookings by workspace type)
• Bottom row: Recent Bookings table (last 5) and Recent Check-ins list (last 5)
• Quick action buttons: Add Workspace, Export Report, View Audit Log
• All sections show loading skeletons while data fetches
• Uses mock data if no API connected