Add comprehensive test framework for BlockPy interface and components

[Copilot]

This PR implements a complete testing infrastructure for the BlockPy client interface and its various features, addressing the need for automated testing to ensure reliability and maintainability.

What's Added

Test Infrastructure

• Jest testing framework with proper ES6 module support, CSS mocking, and JSDOM environment
• Comprehensive mocking system for external dependencies including Skulpt, jQuery, Knockout, FilePond, and browser APIs
• Coverage reporting integrated with npm scripts for tracking test coverage

Test Coverage (8 test suites, 44 tests)

• Unit tests for core utilities, storage wrapper, interface components, editors system, feedback system, and dialog system
• Integration tests for module loading, class initialization, and key user workflows
• HTML template validation ensuring proper structure and accessibility attributes
• Error handling and edge case testing for robustness

Key Components Tested

• Interface subscriptions and UI state management
• LocalStorage wrapper with namespace isolation and error handling
• Feedback system HTML templates and subscription setup
• Dialog system modal functionality and accessibility
• Editor enums and module structure validation
• Utility functions like array manipulation and data processing

Developer Experience

npm test              # Run all tests
npm run test:watch    # Watch mode for development  
npm run test:coverage # Generate coverage reports

Technical Implementation

The testing framework uses a robust mocking strategy that allows testing the client-side interface components without requiring the full BlockPy ecosystem (skulpt, blockly, pedal, etc.). This enables fast, reliable unit tests that focus on the interface logic rather than external dependencies.

Key mocking includes:

• Skulpt Python-to-JS compiler functions
• jQuery/Knockout for DOM manipulation and data binding
• FilePond file upload components
• Browser localStorage API
• CSS imports mapped to identity-obj-proxy

Documentation

Added comprehensive testing documentation (docs/TESTING.md) with:

• Setup and usage instructions
• Test writing guidelines and best practices
• Troubleshooting guide
• Examples for testing BlockPy components

Build Integration

All tests pass, build process remains unchanged, and linting continues to work without issues. The framework is designed to support the existing webpack-based build system while providing modern testing capabilities.

Fixes #141.

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