[Task](greenhouse): Enhance plugin loader for external plugins with error handling and Shadow DOM isolation

[ArtieReus]

Task Description

Enhance the Greenhouse plugin loader to properly support both core (static) and external (dynamic) plugins with robust error handling. Failed external plugin fetches should be gracefully handled without breaking the shell application. External plugins will be safely isolated using Shadow DOM or similar approach to prevent them from breaking or interfering with the shell app.

Sub-tasks

• Enhance usePluginLoader.ts to properly support both core plugins (static imports) and external plugins (dynamic bundles)
• Implement comprehensive error handling for failed external plugin fetches
• Add error logging for plugin load failures (log errors, skip gracefully, allow build to continue)
• Provide clear, user-friendly error messages for debugging plugin issues
• Implement safe attachment mechanism using Shadow DOM or similar approach to isolate external plugins from the shell app
• Add tests to verify error handling scenarios
• Add tests to verify Shadow DOM isolation works correctly
• Document the plugin loading behavior and error handling

Related Issues

• Epic: [EPIC](greenhouse): Dynamic External Plugin Integration System #1624

Additional Context

This task focuses on making the plugin system production-ready by ensuring that external plugin failures don't crash Greenhouse. The error handling should provide clear feedback to developers while maintaining application stability.

External plugins will be attached using Shadow DOM (shadow root) or similar approach to provide style and DOM isolation, preventing external plugins from accidentally breaking or interfering with the shell application's functionality and styling.