Add testing documentation

[grmbyrn]

Add Comprehensive Testing Documentation (Vitest & Playwright)

Related issue: [#54]

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Summary

This PR adds full documentation for the project’s testing setup, clarifying how to run, organize, and extend tests for both unit (Vitest) and end-to-end (Playwright) coverage.
The goal is to help contributors understand the testing stack quickly and avoid confusion around configuration or runner conflicts.

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Changes

• 📝 Created docs/testing.md
  A detailed testing guide covering:

  • Overview of Vitest and Playwright usage
  • How to run unit and E2E tests
  • File organization and naming conventions
  • Example configurations (vitest.config.ts, playwright.config.ts)
  • Sample test code snippets (for both unit and E2E)
  • Troubleshooting common issues

• 🔗 Updated README.md
  Added a new Testing section linking directly to the full guide.

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Key Notes

• Clarifies why Vitest excludes Playwright specs (separate runners & runtimes)
• Provides ready-to-use commands for test execution and coverage
• Documents prerequisites (e.g., npx playwright install, npm run dev)
• Helps onboard new contributors faster with clear, reproducible examples