Introduce Playwright E2E testing infrastructure

[Copilot]

This PR introduces comprehensive end-to-end (E2E) testing infrastructure using Playwright to ensure the application works correctly from a user's perspective.

Changes Made

🧪 E2E Testing Setup

• Added Playwright dependency: Installed @playwright/test as a dev dependency
• Created E2E test directory: Added e2e/ directory with initial test suite
• Playwright configuration: Added playwright.config.ts with optimized settings for CI/CD
• Test script: Added pnpm test:e2e command for easy test execution

📋 Test Coverage

The E2E test (e2e/app.spec.ts) verifies core functionality:

• ✅ Application renders correctly at root URL
• ✅ "Hello, world!" heading is displayed prominently
• ✅ Page title shows "MLack - Real-time Chat"
• ✅ Chat interface elements are present and visible (message input, send button, messages area)

🔧 CI/CD Integration

• GitHub Actions workflow: Added dedicated E2E test job that runs on every push/PR
• Browser installation: Automated Chromium browser setup in CI
• Test artifacts: Configured automatic collection of screenshots and traces on failure
• Parallel execution: Unit tests and E2E tests run in separate jobs for efficiency

📚 Configuration Updates

• Vitest exclusion: Updated vitest.config.ts to exclude E2E tests from unit test runs
• Git ignore: Added Playwright test artifacts to .gitignore
• Documentation: Updated README.md with comprehensive testing section and project structure

🖼️ Application Preview

Usage

# Run E2E tests locally
pnpm test:e2e

# Run unit tests (unchanged)
pnpm test:run

# All tests pass and maintain existing functionality

Benefits

• Quality assurance: Automated verification that the application works from user perspective
• Regression prevention: E2E tests catch issues that unit tests might miss
• CI confidence: Every PR is automatically tested end-to-end
• Developer experience: Easy local testing with artifacts for debugging failures

The infrastructure is minimal, focused, and follows the project's existing patterns using pnpm, TypeScript, and Biome for consistency.

Fixes #28.

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