Update to JupyterLab 4

[Copilot]

This PR migrates the jupyterlab-plugin-graph extension to support JupyterLab 4.x, addressing the deprecation of private APIs and updating all dependencies.

Changes Made

🎯 Core Migration

• Updated dependencies: All JupyterLab packages upgraded from 3.x to 4.x
• API Migration: Replaced deprecated app['_pluginMap'] with hybrid approach using public Lumino APIs
• Plugin Access: Created createPluginsMap() function that:
  • Uses public APIs (listPlugins(), getPluginDescription(), isPluginActivated())
  • Falls back to private _pluginMap for full metadata (requires, optional, provides)
  • Gracefully handles errors if private API access fails

🔧 Technical Improvements

• TypeScript Configuration:
  • Updated target to es2018 and added proper lib configuration
  • Added skipLibCheck: true to resolve dependency type conflicts
• React Compatibility: Updated React types from v17 to v18
• Build System: Replaced jlpm with yarn for consistency
• Code Quality: Fixed ESLint/Prettier formatting issues
• Cytoscape Fix: Removed invalid padding-bottom style property

📦 Version & Documentation

• Version Bump: Updated to 0.3.0 to reflect major compatibility change
• Requirements: Updated README to specify JupyterLab >= 4.0 (was >= 2.0)
• Changelog: Added comprehensive migration notes

✅ Testing & Validation

• TypeScript compilation passes without errors
• Extension builds successfully (both lib and labextension)
• Plugin metadata extraction validated with unit tests
• ESLint checks pass
• No runtime errors in generated JavaScript

Migration Strategy

The extension maintains functionality by using a hybrid approach:

1. Public APIs First: Attempts to use official Lumino APIs when available
2. Graceful Fallback: Falls back to private _pluginMap for complete plugin metadata
3. Error Handling: Gracefully degrades if private API access fails

This ensures compatibility with JupyterLab 4.x while maintaining full functionality until official APIs are available for plugin metadata access.

Breaking Changes

⚠️ Minimum JupyterLab version is now 4.0 (previously 2.0)

Fixes #39.

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