Add E2E tests (Cypress/Playwright) for critical user flows

[pcastelo]

Problem

No E2E tests validate the full user workflow (frontend → backend → mock cluster). Integration issues between frontend and backend only surface in manual testing.

Acceptance criteria

• Set up Cypress or Playwright for E2E testing
• Create test scenarios for critical flows:
  • Login → Dashboard loads cluster status
  • Navigate to migration wizard → Select products → Analyze → Review resources
  • Settings → Add 3scale source → Verify it appears in explorer
  • Chat → Send message → Receive AI response
• Mock backend API or use Quarkus dev services for E2E
• Add E2E step to CI (can run on schedule, not every PR)
• Generate E2E test report as CI artifact