Add Vitest test suite with CI integration

[Copilot]

Adds comprehensive test coverage for the PDF viewer web component using Vitest and @open-wc/testing.

Test Coverage

• Component tests for pdf-viewer, pdf-toolbar, pdf-canvas, pdf-sidebar, and theme configuration
• Helper utilities for text normalization
• Integration tests for complete user workflows
• Mock implementations for PDF.js worker and Canvas APIs

CI Integration

• GitHub Actions workflow runs tests on push and pull requests
• Uses happy-dom for lightweight DOM testing
• Coverage reporting configured with exclusions for styles and build artifacts

Documentation

• Added testing section to main README with basic commands
• Removed verbose test/README.md to keep documentation focused

yarn test              # Run all tests
yarn test --watch      # Watch mode
yarn test:coverage     # With coverage

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