feat: add Playwright E2E testing infrastructure with tree listing

[WilsonNet]

Summary

This PR adds comprehensive Playwright E2E testing infrastructure to the dashboard project. The implementation includes:

• Playwright configuration and setup with proper TypeScript support
• E2E tests for tree listing page with robust selectors and comprehensive coverage
• Strict TypeScript configuration specifically for e2e tests
• Environment management with staging as the default test environment
• Documentation and configuration files for developer onboarding
• Proper artifact management with appropriate .gitignore entries

Changes

🔧 Framework Setup

• Added Playwright as dev dependency with proper configuration
• Created playwright.config.ts with environment support
• Set up strict TypeScript configuration in tsconfig.e2e.json

🧪 Test Implementation

• Renamed and cleaned up test file from basic.spec.ts to tree-listing.spec.ts
• Implemented comprehensive tests covering: tree listing load, time interval changes, table size changes, search, navigation, and pagination
• Created proper selector file (e2e-selectors.ts) with robust element selectors
• Removed obvious comments for cleaner test code

📋 Configuration & Environment

• Updated package.json with single e2e command and e2e-ui command
• Added staging environment as default in .env.example
• Added comprehensive documentation to README.md explaining how to run e2e tests
• Updated ESLint and TypeScript configurations to support e2e tests

🗂️ Git Management

• Added Playwright artifacts (test-results/, playwright-report/) to .gitignore

Benefits

• Better test coverage: End-to-end tests for critical user flows
• Improved reliability: Tests run against real environments with proper selectors
• Developer experience: Clear documentation and streamlined commands
• Maintainability: Proper configuration separation and strict TypeScript settings
• Consistency: Staging as default environment for consistent testing

Usage

# Run tests against default staging environment
pnpm run e2e

# Run tests with UI mode for debugging
pnpm run e2e-ui

# Run against different environments
PLAYWRIGHT_TEST_BASE_URL=https://dashboard.kernelci.org pnpm run e2e  # production
PLAYWRIGHT_TEST_BASE_URL=http://localhost:5173 pnpm run e2e           # local