Add UI end-to-end tests with Playwright

[sozercan]

Summary

Add end-to-end tests for the web UI using Playwright. The @playwright/test dependency is already installed but no Playwright config or e2e tests exist yet.

Motivation

The frontend currently has unit/component tests (Vitest + Testing Library + MSW) but lacks e2e tests that exercise full user flows through the browser. E2e tests would catch integration issues between frontend and backend that unit tests miss.

Scope

• Add playwright.config.ts to the frontend
• Set up test fixtures and helpers (authenticated session, mock cluster data)
• Add e2e tests for core user flows:
  • Navigation and layout rendering
  • Model deployments list and detail views
  • Create model deployment flow
  • Provider configuration views
  • Error states and loading states
• Integrate with CI (GitHub Actions)
• Add documentation for running e2e tests locally

Notes

• A root-level test:e2e script already exists: bun run --filter '@kubeairunway/frontend' test:e2e
• MSW is already set up for mocking API responses
• Controller has its own Go-based e2e tests in controller/test/e2e/ — this issue covers UI-only e2e tests