feat: fully functional Admin Panel (#29)

[d1narim]

Summary

Replace the placeholder AdminPanelPage with a fully functional admin operations panel.

Backend

• adminAuth.js: Admin role verification middleware
• adminRoutes.js: 17 endpoints for all admin operations
• adminController.js: 16 controller functions
• adminService.js: Full mock data services with pagination
• Prisma schema: Added Dispute, AuditLog, PlatformConfig models

Frontend

6-tab admin panel with dark theme:

1. Dashboard - Metrics cards + trust score bar chart
2. Users - Search/filter table with suspend/reinstate/ban/delete
3. Moderation - Flagged jobs queue with approve/reject/escalate
4. Disputes - Queue with rule-in-favor workflow
5. Controls - Toggle switches with confirm dialogs
6. Audit Log - Filterable paginated log

Closes #29

Wallet (TRC20): TUuRaG77MWb3HaW4o9G1Crsm9fY4ofeYwL