[Frontend] Replace mock data with real API calls (donors, donations, accounts)

[nourshoreibah]

These pages render hardcoded arrays instead of calling the backend:

• src/app/donors/page.tsx:20 (mockDonors)
• src/app/donations/page.tsx:18 (mockDonors/mockDonations/mockProjects)
• src/app/accounts/page.tsx:14 (mockUsers)

Wire each to apiFetch (donors :3003, projects :3002, users :3001) following the pattern in expenses/page.tsx. Note: depends on backend donor/donation endpoints (#244, #245).