Replace hardcoded data with API calls in sandboxes and workspaces pages

[kivo360]

Problem

Several dashboard pages use hardcoded mock data instead of fetching from the API:

• Sandboxes page (frontend/app/(app)/sandboxes/page.tsx): Uses static workspaces array with fake data
• Workspaces page: Similar hardcoded workspace list

What to do

1. Replace hardcoded arrays with React Query hooks that fetch from the actual API
2. Add proper loading states (Skeleton components)
3. Add error handling for failed fetches
4. Wire up real-time updates via WebSocket where applicable

The API already has sandbox endpoints (/api/v1/sandbox/*) that return real data.

Difficulty: Medium | Skill: Frontend (React Query, Next.js)